At various times Chrome has been one of the standard "Suggested Apps" that got installed via the usual bundling culprits. I know it was one of the things sometimes installed by OpenCandy.
I have no idea if Google still sneaks Chrome onto peoples' PCs that way. But I believe most of these adware installers first scan your PC to see what you have installed, then offer selections from among what you don't have sneakily installed yet.
So since you manually installed Chrome, you wouldn't see that offer.