So my pool is a PPLNS one with Pay-Per-Last-5000-Shares

Basically I keep seeing my share climb up to rougly 15% at times then drops back down to 10% when big rigs join our pool, then seem to dash off again.  Should that be happening.  I thought PPLNS shares only rose.

Is something going on that I'm not aware off and why would 70 GH's keep popping in and out of our very small (2 gh/'s most of the time) pool?