Batteries do not last forever. Charging capacity and discharge time can diminish over time. Battiery's can also only be charged a certain number of times. This is part of it's "useful life". How you use the camera also affects run time. Constant zooming, using the LCD screen, etc.
Temperature can affect a battery's discharge capability. So if the battery is being subjected to extreme flucuations or conditions, you might notice erratic behavior. Or, maybe its just "tired. Original Canon battery's come with a warranty, if yours is still covered and defective, you might get it replaced by the retailer or manufacturer (Canon) in this case.
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Excellent points by Rick!
Camcorder power demand varies greatly based upon what it is doing including both the electromechanical side (zooming, focusing) and also the purely electronic side where load on the various signal processing and conditioning circuits vary greatly depending upon what is being done (video capture/encoding rate, use of electronic image stabilization, use of view finder vs "panel" display, etc. As the load on the battery changes, the battery state monitor is constantly calculating remaining life and I have noticed that Canon doesn't dampen the displayed results very much in their camcorder (at least not in my XF-400); if you put it in automotive terms the battery remaining display in your camcorder is biased more towards instant rather than average fuel economy 🙂
If it is very quickly jumping around then it is likely that your battery does have an issue or there could be a power contact issue. As batteries age, their ability to source higher current drops rather rapidly and so the estimate of remaining time will vary more rapidly and radically with changing current draw. I can't remember whether the menu in my XF-400 provides a battery health indicator like my EOS 1 DSLR bodies do but that might be something to check. And take a look at the battery and camera contacts because they may need cleaning. Use either high proof isopropyl alcohol or WD-40 either of which is a decent contact cleaner, use a soft cloth and be gentle. You are just trying to remove any surface contamination, NOT the plating!