Gain and Volume are two different methods for adjusting how loud your audio is. While I'll leave specific details to someone more audio-focused, you can think of Gain as an input signal and Volume as adjustment to the output (I think). In practice you may not actually notice much of a difference in what the result is while working with the clips in Premiere, but what it means is that you can adjust both of those things independently.
The Gain menu (pictured) allows you to normalize clips based on peaks or to make specific adjustments to the gain level. This is going to affect an entire clip's audio level.

Volume is an animatable property on the clip that you can adjust in Effect Controls or in the timeline. A volume level of 0 does not mean the volume is set to 0dB. It just means that you haven't adjusted the volume either up or down from its starting point. (It's the same with the gain.)
In your case you've added gain to the clips, perhaps in the process of normalization, so depending on the clips peak value it the gain would be adjusted differently for each clip. Then if you wanted to make fine tuned adjustments to clips via keyframes on the volume you could also do that.
