There must be a mistake, as a 5770 is not going to be pulling 275-300 MH/s without nitrogen cooling and an extreme overclock + volt mod.
Most likely the load keeps shifting back and forth between them, and the output displays are out of phase with each other
(so instead of seeing 100 MH/s + 100 MH/s at exactly the same time, or 125 MH/s + 75 MH/s, they're updating out of synch with each other.)
My 5770 clocked in the 900's gets in the 200-210 range. Also a tip - lower your ram to 300 or 302, it will cut power, heat, and increase hashrate.