Exactly one gpu always slower in multi GPU rigs

[vex]

Powercolor 280x, 13.11, win7 ultimate 64bit, cgminer 3.72, one cgminer instance per GPU (because I want per card hashrate monitoring from the pool side)

In all my multi gpu rigs, one card is always couple of percent slower, i.e. all cards are 720khash/s, except of one which is 670-90khash/s

one cgminer instance per gpu. Exactly the same cgminer.conf with all gpus, and all gpu’s are identical.