Did they definitely replace the battery?

Hello all,

I took in my iPhone 6S to the apple store for battery replacement today.

I handed my phone to them with about 23% battery charge.

I received it back with 26% battery charge which seemed rather odd, the assistant did not know why but said it was fine.

I checked the Battery Health (beta) and it says "Maximum Capacity 100%"

Does this mean they definitely changed the battery? or can they just reset the battery statistics so it says "Maximum Capacity 100%"

My battery wasn't completely shot before so I'm not sure whether it will be obvious to me just by using it.

Thanks!