My guess is that the Chrome team feels pretty good about the cross-platform framework bits of Chrome and don't expect to make major changes to it for a while. They're still going to have a lot of Win32 code in there to support newer versions of Windows since it is extremely unlikely they are going to migrate it fully to WinRT (or anything other than basic Win32 APIs), so it isn't like they'd be able to drop a huge amount of code from ongoing maintenance by dropping XP support anyway. It's probably mostly just a bit of extra release engineering work to them at this point.
Given this, the cost of continued XP support is pretty minimal and the PR from supporting XP longer than Microsoft does is good for them, so why not? I wouldn't expect them to really drop XP support until they eventually go 64-bit only on the desktop (which may be what the continued sunset-date they are giving is a hedge against).
No matter what the reason, this is good for adoption of newer web technology. If you do get stuck with laggards still running XP, tell them to use Chrome. They might not want to bother, but at least you're giving them a better potential solution than "fuck off".
Given this, the cost of continued XP support is pretty minimal and the PR from supporting XP longer than Microsoft does is good for them, so why not? I wouldn't expect them to really drop XP support until they eventually go 64-bit only on the desktop (which may be what the continued sunset-date they are giving is a hedge against).
No matter what the reason, this is good for adoption of newer web technology. If you do get stuck with laggards still running XP, tell them to use Chrome. They might not want to bother, but at least you're giving them a better potential solution than "fuck off".