Glide has their own google analytics account so they keep the basic stats for all apps. Pro would allow you to get more detailed information with your own analytics account.
I’m not sure why it would jump like that unless most of the users were from the very beginning of the past 30 days, so it only showed a few users, then the next day you had several new unique devices using the app. It will be a number that will fluctuate up and down, but most importantly it counts over the past 30 days.