According to Motorola, Google makes it very difficult for Android updates to come out on a timely basis because of the way Google operates and how they select the hardware for its Nexus devices. Keep in mind that this is coming from Motorola, soon-to-be subsidiary of Google. And perhaps what Motorola said was meant for public consumption - that Google will not favor Motorola going forwards. Words are just that, words, and we'll just have to see what the actions are.
Still, this is an interesting insight into what how Google operates as far as updating Android goes. It's interesting because the perception that Android is an open platform is now out the window. Android is an OS that is still "Android" as long as Google's stipulations are followed. Once this OS is forked, it's no longer called Android and it's entirely something different.
Now, what does Motorola mean?
It's related to how Google picks the hardware and specs for its Nexus devices. Google picks one device from the field. It's a field that has different configurations - memories, CPU, and other components. Once Google settles on one, it makes everyone else's life more difficult because they would have picked vastly different hardware.
Hence, the delay as other device makers have to make Android work with their hardware. On top of that, Google has optimized Android for its chosen hardware, forcing others to make do with what they can.
As we well know, Apple only has to deal with its own iOS devices - so far, it's one chip a year. On the other hand, Microsoft doesn't have a "Nexus" device, a showcase device for others to follow so it doesn't have this issue. We have to wonder how Microsoft deals with the multiple chipsets - whether it optimizes the OS for each and every single device or does it "just make do". It's like Microsoft's partnership with Nokia involves Nexus-level attention.
More at Droid-Life.