the AP is a mikrotik r52hn on a rb333 or 433 set to B/G/N with B rates disabled.

But if the CPE is set to onlyN, why does it use the G rates still?

An ubiquiti device for example that is an 802.11N device will not use G rates at all if it sees an N AP.

this page will help to understand what rates are used for each band setting:
http://wiki.mikrotik.com/wiki/Manual:In ... Rate_table