That's not necessarily true for all smartphones. The release cycle works something like this: Google releases core Android to manufacturers once it's ready. The manufacturers then adapt Android to their models that are still considered new enough and capable enough to be updated. How they determine that, is up to them. Then, if it's a carrier-based device, the manufacturer's rom is released to the carrier so they can modify it to their purposes. The less they do to it, the quicker it goes out to customers ... theoretically.
However, no manufacturer, carrier or even Google themselves release an update to all devices on the same day. They all pick groups of devices and send out an update or a notice according to a schedule. Again, how they determine the groups and the schedule are anybody's guess. Nexus devices will get the updates right from Google so they get them first. DE (Developer Edition) phones should also get updates very quickly as they run more or less pure Android. After that it's really up to the carrier or manufacturer.
It is quite possible that if you are on the end of a cycle with an unlocked device, that the carrier-branded models at the beginning of their cycle will get the update before you, but it's not a universal situation with all Android devices.
