It appears that software updates for even Nexus devices have to undergo some sort of carrier approval.

See Google employee Jean-Baptiste Queru's Google+ post and follow-up comments on this topic: https://plus.google.com/112218872649456413744/posts/LG8VK9NN...

Relevant snippet below:

As for the updates to flagship devices (including the Nexus phones), even though Google tries to get builds ready for all devices at the same time, operator approval varies from one operator to the other and from one device to the other. The 8-month rule of thumb doesn't apply to flagship devices, since all phases are designed to happen at the same time for those, but there's still a bit of operator certification and approval at the end that can't overlap with anything else, and that's what explains the slight device-to-device variance even for flagship devices.