I really think 14GH is the capacity of G28 skein.
But there s Something wrong the "dacc" is around 80 to 90% on the G28 with difficulty managed by suprnova. This is too low, in comparison the X10 on same pool is around 99% explaining X10 is at nominal hashrate.
I tried to force difficulty on suprnova skein for the G28, but with no luck so far the best i achived is an average 12,8 GH/s which is still far from 14GH.
My guess is it is pool related. Though i contacted them twice by their support form and on twitter and NO answer since 48H.
I have a similar experience, pools are reporting 10-20% less hashrate than advertised. Skein bounced between 11-14 Gh with an average around 12.5-13 I'd say. Myr-Gr was similar and averaged around 25-26 Gh.