Is my battery glitchy?

So after getting my sister the 6+ she gave me her old 5S. However, it seems like the battery is draining fast. I took it to Apple and they told me the battery is still at 96% capacity. Upon using the ibackupbot program I saw that my battery had still 1500mah out of 1550 left. It seems like Apple is right but when I looked at the number of charges I saw 351. The warranty expires on the 25th so it seems right but I can't understand why the battery is in good condition after so many charges. I know Apple has said that the battery can degrade to 80% capacity after 500 charges but this doesn't correlate with the status of my battery right now. Do I just have a really good battery or is the battery somehow reporting the wrong information? I've searched the forum and have seen some users report their battery at about 85-90% capacity after ~350 charges.