Well.. the Nexus devices are a bit of an oddity.... Typically for OEM phones, like a Note, Google releases the Android source, it goes through the OEM for development (which also includes carrier modifications), the software is given to the carrier for qualification, and then it is published. The carrier then decides which phones get what version.
The Nexus 6 cuts out the middleman in a sense, since Google handles all the software. But you still need to have your phone 'blessed' by the carrier... but the update itself still comes from Google. So yes, what you get and when you get it is determined by the SIM that you have loaded in your phone. There are a couple of different versions kicking around at the moment... 5.0 (release), 5.0.1 (the update from December). Then things got a little messy with the release of 5.1... the addition of VoLTE seems to have caused a bit of a scramble as there are currently 3 different versions in the wild. LMY47D, LMY47E and LMY47M.... I'm not sure who got D, but E was really for Verizon and M is meant for T-Mobile.
Funny thing is that any of these versions are for the most part compatible with every Nexus 6.. other than the VoLTE additions (maybe)... there MAY be some variation there between carriers and versions.... but it gets messy. For instance, M is for T-Mobile, but the radio included appears to be the same as the D release... E, for Verizon, uses different, newer radio firmware, presumably to support VoLTE on Verizon's network. BUT.... there are some people on T-Mobile that loaded E, with the Verizon radios, that say that VoLTE is working.
It's kind of a odd mess.... and I suspect that the next updates (5.1.1, 5.2? not sure) might see the different forks converge again to a single release... at least until T-Mobile incorporates Wi-Fi Calling...
