Note that this is an estimate. Judging by the 10 days 6 hours in standby mode, something in your usage of the phone is telling it that you use a lot of battery-hungry apps or keep the screen on often when not in standby, which leads to this lower estimate.
The estimates change based on past usage, I noticed, not on current circumstances (stamina mode excepted, it will change the estimate). Perhaps you've recently used some battery-guzzling app and so it adjusted the estimate based on that.
Use it and see how long it lasts, then see the information B. Diddy talked about (apps power usage, screen on time, etc) to judge how and where that battery went.