Massive difference in winrate? You are talking about fewer than 500 total games played. Nothing massive at all.
Interesting article regarding just this (from 2013):
I know the article is dated, but Blizz doesn't really release much of this type of data. I can't imagine much has changed in this regards.
You guys said at Blizzcon that there was a slight advantage in going first over second. Is this still holding true, and are there any matchups which vastly differ from the median?
Eric: It's still true. Actually, I think for every class now there's a small advantage to going first at every level.
Ben: It's about three percent better to go first over second. It's only a tiny bit better to go first at the lower levels of play.
Eric: And that being said, are there some decks which are built around going second, and might have a small advantage for going second? Yeah, that's certainly the case, but the advantage for going second in those decks I would imagine is less than one percent.