I don't know how trustworthy that actually is, it will say I've used some apps for only a couple minutes that I was actually using for around 30 minutes. The usage number should be accurate for screen time as long as you weren't listening to audio with the device locked, using that you can achieve usage times of 30+ hours.
The times are from the last 24 hours (or up to 7 days), they don't line up with the last time the phone was fully charged. So no direct comparisons could really be made given two different starting points (unless the last time you fully charged happens to be exactly 24 hours ago at that moment).
Aside from the timing being different, if the phone was doing something in the background then it was using up resources, right, so why wouldn't we want to know about that time (given that it wasn't just idle not doing anything).
Snapchat does the same thing. Thats cause a few apps aren't optimized for iOS 9. Its smart to just kill the app any time you're not using it or it'll drain your battery in the background.
So it's logging the time that a app remains in background until the need for RAM purges it? I think that's just going to be confusing for a lot of people, maybe if it actually included a separate percentage of battery used while a app is backgrounded which should be small with only location service and background app refresh bringing the possibly of it being larger. I also wish they had included a option to only show battery percentage used over the course of a full 100% charge, that would cause less people worrying about a app being listed as using 60% over 24 hours not realizing that's because they didn't use their phone much that day other than that particular app, the number only correctly correlates to actual battery percentage used if you start with 100% at the start of a 24 hour cycle.
No, it's not the time that it is in memory, but actual time it is doing something in the background. Normally apps are limited to do that for a few minutes, and background refresh can allow them to do that for a few minutes at different times, but then there are also features of some apps (like those with navigation capabilities, VoIP capabilities, and some others) to run in the background for longer periods of time than a typical app.
