When investigating time (mis)keeping on the D-Link DI614+, I found exactly the same thing there. Walking the strings of the firmware reveals a hardcoded list ntp servers and from observation it looks like they walk down the list, primary ntp servers first, to get the time.
The D-Link firmware is cobbled together from quite a few different libraries. It maybe the code exists in a library both systems use or the systems are re-badged from a common source.
How many others then???