It's not uncommon to get somewhat faster speeds than what you are promised, especially if you live in an area where the infrastructure for higher speeds is there and not well utilized.
That said, getting almost 4x your advertised DL speed is a little worrisome. I would suggest verifying with Comcast that you are being billed for 25mbps down. If so and you still are getting 90mbps down on ethernet, enjoy it while it lasts.