The more i look at it, im thinking the shares set up skews towards high powered miners. Still to prove it with some maths and tests. Last exam is on Tuesday. After that I can have a proper look.
I believe the use of "seconds since round start" is the problem, really. It should really be a function of shares since round start, however that would mean a lot of extra computational effort to keep track of. Use of seconds is a decent approximation, and is fair over the long term (assuming perfect uptime).