It happens with Apple OS updates too, Apple just have sufficient market clout to tell networks they must complete the testing in the week between the gold master release and the public release.
GSM isn't some magic specification. It's entirely possible for a crappy radio firmware to cause significant disruption to a network it connects to (indeed, I've seen a pre-release firmware from a mid-tier Android manufacturer that managed to cause a reboot in every cell tower it connected to from one of the UK's networks). That's an extreme example, but carriers frequently do testing on that basis, and it often holds up the European releases of Android software updates.
It happens with Apple OS updates too, Apple just have sufficient market clout to tell networks they must complete the testing in the week between the gold master release and the public release.
GSM isn't some magic specification. It's entirely possible for a crappy radio firmware to cause significant disruption to a network it connects to (indeed, I've seen a pre-release firmware from a mid-tier Android manufacturer that managed to cause a reboot in every cell tower it connected to from one of the UK's networks). That's an extreme example, but carriers frequently do testing on that basis, and it often holds up the European releases of Android software updates.