Hello newkirk,
Are you using the original ePMP1000 Cambium antenna or the newer Cambium 90/120 sector (C050900D021A) antenna with these APs? The ePMP1000 platform has a lower tx power capability in the lower band (http://community.cambiumnetworks.com/t5/ePMP-Configuration-Management/Tx-Power-Configuration/m-p/53650#M24), but it does not explain the difference you see. Could be a function of the antenna for the frequency and angle to the SMs.
Have you tried the same experiment with an ePMP2000 and newer Cambium 90/120 sector antenna?
Regards