MBP Retina Full charge capacity has mysteriously increased ??

I bought a Macbook Pro retina from ebay a couple of weeks ago. It is the mid 2014 13" model. I bought it knowing it had staingate so I was expecting to take it to the genuis bar to get a new screen fitted.

When i got there they told asked if there was any other problems. i told them the battery percentage just stays on whatever it was at after i remove the charger , and doesnt reflect the actual percentage remaining, even hours after use. The apple genius said he would add put a note to run the laptops battery through KOGS.

The top lid and screen is brand new, the battery percentage is fixed. Not only that but my full charge capacity on my macbook retina pro has mysteriously increased by over 300 mAh.

Obviously i am ecstatic, but wanted to know how this happened. Below on the right is my current Battery information and on the left is the battery info from the ebay listing, which i confirmed was correct once i received the mbp.

Anyone have any clue on whats going on ???

My cycle count is consistent so im sure the battery wasnt changed