Routerboard frequencies random or anything else?

Hi, I hawe 2 RB411: one as APbridge, the other as station WDS.

AP bridge settings (wireless):
Mode = ap bridge
Band = 2.4GHz-B/G
Frequency = 2457
Scan List: none
DFS mode = no radar detect

Status: frequency = 2427 ???

Station WDS settings (wireless):
Mode = station wds
Band = 2.4GHz-B/G
Frequency = 2472
Scan List: none
DFS mode = none

Status: frequency = 2427 ???

Everytime I reboot I have e new frequency, but different from both configurations.
Maybe wrong setting in dfs mode?
How can I set manually the frequency and how can I set the best frequency (in automatic)?
Thanks.