Interesting. I did not know that. It seems someone else is setting the mtu (I rebooted my router to be sure it was not a lingering test case), which is why I believed it would use 1500:

admin@RT-N66U:/# cat /proc/sys/net/ipv6/conf/default/mtu
1280
admin@RT-N66U:/# cat /proc/sys/net/ipv6/conf/all/mtu
1280
admin@RT-N66U:/# cat /proc/sys/net/ipv6/conf/br0/mtu
1500