Hi...Interesting, may be Asus didn't make other channel available or may be use for specific use.
Probably to meet European regulations.
According to the table above (not sure how up to date it is), the 5Ghz indoor usage should happen from channel 32 to 68.
Channels over 140 appear tagged for SRD usage (140 and 142 also as DFS).
So it looks like that any channel tagged for SRD usage was just blocked by Asus.
On the 380.70 firmware, the top limit was/is channel 165.