Yeah, and there's always someone in April who's on pace to hit 80 home runs for the season. That said, I think the trend toward Chrome, and webkit browsers in general, is undeniable.
Linear extrapolation in general is unwise, but with market share it is particularly foolish. I'd bet the opposite, that the gap between the two browsers takes longer than a year to vanish.
In South America, where these stats show Chrome in the lead, the growth seemed to be accelerating up to and beyond the crossover point (the same in the few individual countries where they're leading). What about marketshare particularly should make this unusual or unexpected?
I'd assume some kind of tipping point effect as it becomes "normal" not to run IE and/or to run Chrome that could lead to such acceleration since the barriers to adoption are so low.
Perhaps there's a distribution deal or popular website in South America that requires Chrome to function best (Orkut?). Or perhaps, as you said, it's just a network effect.