Battery health

Health can drop fast at times (or what is perceived as fast). Unless it drops to 80% it is not and will not be considered an issue. If it drops to 80% during the warranty period (or during the time you have an active AppleCare+ plan) then it would eligible for replacement.

Batteries lose about 1% capacity per month, on average. And, since you bought a phone that must have been sitting on a shelf somewhere for a year, the battery was probably already aged a bit from not having been charged in so long.