5 GHz Downlink Data restriction when using DFS

We are running 16.1 on a 5GHz PMP 450i and have just noticed that the range of Download Data is restricted to 15%-75%. After some more checking to see if this is model dependent, I found a 16.1 5GHz 450 (non-i) which is also restrictred to 75%. The common setting for these two radios is the usage of a DFS channel, which I presume is causing the restriction.

Does using DFS make it so you can't use more than 75% Download Data?

I noticed this quite some time ago and when I asked I was told it had to do with the band and how it was certified with the FCC. Other bands/radios typically allow up to 85% down/up ratio, like on the PMP450i 900 or 3ghz.