Just to clarify, what you are referring to appears to be Exposure Compensation (E.C.).
That's used to bias the AE or "auto exposure" modes (Av, Tv, P) in 1/3 or 1/2 stop increments, usually to correct for subject tonality and other things that tend to "fool" the meter in the camera.
First, double check that you aren't accidentally using "A+" mode, which is very highly automated. Be sure you've set the mode dial to "Av".
Also be sure you don't have Auto Lighting Optimizer or Highlight Tone Priority enabled. Those will automatically override some of your settings and can cause unexpected changes to your exposure selections.
When you're setting E.C., it shows how much you have dialed in on the top LCD of the camera,within the viewfinder and in the menu. The "lock" is used so that the amount of E.C. you've dialed in isn't accidentally changed while shooting, by bumping the rear (quick) dial. That DOES NOT lock the metering system (you typically wouldn't want it locked, in many situations where it's necessary to use the AE modes).
Once you return to shooting the metering system will now constantly change the settings as needed, in response to different light levels it's detecting. However, it continues to be biased by whatever amount of E.C. you dialed in. There is a separate AE Lock (*) button on the camera, that can be used for individual images, at times when needed.
Actually your old camera worked much the same way, except that the Rebel series lack a direct means of accessing E.C., via the dial on the back of the camera ("quick dial"). Instead you had to press a button and then use the top (main) dial. There also isn't a top LCD panel displaying the settings, on all but one of the Rebel series (the 760D or T6s has a top LCD and uses controls more similar to 60D/70D).
When in Av mode, as you say you like to do... all that's "locked" is the aperture you've set. Auto exposure then adjusts shutter speeds as needed, to produce what the camera considers a "correct" exposure. (Except when you press the */AE Lock button.)
In Tv mode, the shutter speed is locked.
In P or Program mode, the camera can change both shutter speeds and aperture, as it calculates is necessary.
Today's cameras also have Auto ISO, which I generally don't recommend using in combination with any of the AE modes. It just overly complicates things, rather unnecessarily.
When you use M (Manual) exposure mode, the same metering readout scale is displayed, but it now serves a different function. Centering the indicator on the 0 mark will give you essentially the same exposure that the camera would use if it were in one of the AE modes, except it's achieved with your choice of ISO, aperture and shutter speed. In this case you won't see your setting changing... they are truly locked in. In M mode you can bias slight over or under exposure, too... much like using E.C. And the bias will be shown on the metering scale, in a similar way.
If using Auto ISO with M mode you are no longer doing manual exposures. It's now just another form of auto exposure, where you can lock in both aperture and shutter speed, but the camera still handles exposure by adjusting the ISO.
Also, if using ETTL flash you're once again getting a form of auto exposure even when the camera is set to M (and Auto ISO is off), though this time the camera is setting exposure by controlling strength of the flash's output.
You have gone from a fairly automated, entry-level camera to the most pro-oriented, advanced and complex APS-C model Canon has ever offered. To get the best out of the camera, you'll need to spend some time with the user manual, learning to use it. (500+ pages! Those manuals just seem to keep getting bigger and bigger!)
I would also highly recommend you get and thoroughly read one or more of the guide books written about the 7DII. You can find these many places. I did a quick search on Amazon and found quite a few are being offered now. Although I have been shooting for several decades and consider myself fairly knowledgeable about cameras, I always make a point of getting at least one of these guide books whenever I get a new camera, to shorten the learning curve and help me get up to speed using it well as quickly as possible. These books nicely complement the user manual, and while I'm sure there are many good ones, I have personally used and can recommend those by David Busch, Charlotte Lowrie and Michael Guncheon. If you prefer, there are DVD and online tutorials available.
If you haven't already read it, I also strongly recommend Bryan Peterson's "Understanding Exposure". It's more general in nature, an excellent and thorough explanation of many of the mysteries of photography that might apply to practically any camera ever made. I consider it a "must read" for anyone wanting to advance beyond highly automated "point n shoot" photography.
Hope this helps!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
