Yes, please tell us which camera model you are using. To fully understand the camera behavior, you must have a firm grasp of how the Exposure Triangle works.
I think I know what you are seeing. The behavior I think you are observing is normal for certain entry-level camera models. This would include the Rebel T7 and its predecessors in the USA. This would include cameras with model numbers that contain four digits, ####D.
The user interface for these entry-level models seem to have been designed to provide the user with a simple set of controls. The user seems limited to manually setting [exposure] at full stop intervals when you are using M mode. The camera almost encourages the user to set ISO to Auto [when using M mode], which is my recommendation to do when using one of these models.
For example, this [restriction[ means you are limited to setting ISO to 100, 200, 400, 800, etc. For shutter speed you are limited to setting to full stop increments, such as 1/100, 1/200, 1/400, 1/800, etc. You will experience the full stop limitation when setting Aperture, too. I seem to recall that this is the default behavior. This limitation reduces the complexity of setting exposure. The user is limited to a couple dozen or so choices, instead of dozens of choices.
The cameras also have the option to allow setting exposure in Full stop, 1/2 stop, or 1/3 stop intervals. I believe the default setting is a full stop, but it [seems to make] no difference to the user. If you set it to 1/3 stop increments, the user still seems limited to only setting exposure at full stop intervals.
The menu setting seems to control what the camera is able to do automatically, leaving the user stuck with only setting full stop intervals. ( But, there is a work around that i will describe in a moment. )
However, if you set the camera to any other mode that allows the camera to control [at least one leg of the Exposure Triangle], then the camera is able to automatically adjust exposure at full stop, 1/2 stop, or 1/3 stop increments. The camera behavior is controlled by the setting I described above, while the setting seems to have no effect on what the user can do.
Actually, the setting does have an effect on what the user is able to do. This is where a good knowledge of the Exposure Triangle comes into play. To set the exposure in 1/3 stop increments the user must use Exposure Compensation. The menu setting also controls how much control over exposure the user has, but that control is only available to the user indirectly.
The ability to set Exposure Compensation is only available in P, Tv, and Av modes because these modes allow the camera to automatically one leg of the Exposure Triangle. When the camera controls at least one leg of the Exposure Triangle, then the user is able to dial in Exposure Compensation.
This “work around” of using Exposure Compensation is how the user is able to control exposure settings at intervals of less than a full stop.
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