I think Chelsea initially leaked the offer.
Mourinho with his "I'm not talking about other teams and their players" yet saying absolutely everything he wanted to say about Stones. Then having Cahill and his other players talk to the media about how great Chelsea is, how great John is and how great John could be if he signed for Chelsea.
This then backfired for a couple of reasons. First, we can no longer buy out Stones' sell on fee from Barnsley. However slim a chance of it happening before was, it's impossible now. That means we're definitely going to want more to make up for the X% that we'll have to give to Burnley.
Secondly, we have now taken this discussion and by not completely ruling any chance of him leaving out, allowed City and United to enter a possible bidding war, driving his price up even more.
Notice how Chelsea didn't say anything about the second bid. They want to keep it quiet now because they know it's in their best interest to do so. We're the ones who "announced" that we have rejected a second bid from Chelsea, essentially saying "look, Chelsea are offering £26m for Stones, anyone care to beat that?"
He may not necessarily be for sale but we're making sure we'll get the best price possible if he does go.