phone vendors and carriers both same issue.
So here is what happens.
1 - Google release new version of android, its not specific to any phone so doesnt include drivers, carrier apps etc.
2 - Phone vendors e.g. samsung, sony, htc etc. Then will make new versions of rom's for their devices based on what google releases, they will only do this for recent phone model's when a phone gets old enough they stop this process.
3 - The carriers 'after' the vendors do new versions may then ammend these again for their own purpose, typically branding and adding their own apps. They usually support the same phones the vendors do.
So as you can see there will be an obvious delay between when google release and when end users get the updates,when samsung did 5.0 for the S5 e.g. 5.1 was already released by google. Second old phones dont get the updates and unless you prepared to root your phone there is no way to get newer rom's on those old devices. Rooting your phone typically voids warranty and the custom rom's are usually more problematic than official one's.
There is also often many minor releases google release to fix minor issues and patch security holes, and vendor's typically skip over these.
If you buy your phone from a carrier, then the carrier handles the updates so you have to wait for #3.
If you get it unlocked not tied to a network then your updates come from #2.
Some models such as the nexus phones will get from #1 and as such get more frequent updates.
Edited by Chrysalis (Mon 18-May-15 17:40:21)