Some set-top boxes have an architecture where they keep two copies of the system software so they can revert to the previous version if it all goes wrong.
It's a lot easier for the set-top box software to be tested before it is downloaded, plus they own the broadcast bandwidth, so they can use as much as they want. The cable (or satellite, or IPTV) company is operating in a highly constrained environment, they control what devices are connected to their network, they know exactly what hardware is out there. So they can test the software on every different type of hardware that they have deployed before it goes out. This may take them a couple of months. Of course they have a big incentive to make sure their updates don't break the box as every call to their customer support call centre costs money, and if they brick the box, a truck roll costs an arm and a leg!
So updating set-top boxes in a closed environment is a significantly different (and easier) problem to a general "update Linux on any embedded device".