For 802.11n to hit speeds around 270, it has to use wide channels (40mhz) compared to the standard 20mhz space. Per the spec, a router has to stop using wide channels if it senses a conflict, and this is highly likely to happen if you are on 2.4ghz and in an area where your laptop can see other wireless networks.

It's possible your router is trying to use wide channels, and the Windows drivers see that and report the theoretical speed possible, whereas the OS X side may be reporting the actual speed due to dropping down to the standard channel usage.