The figure on the web site is only an estimate, back-calculated from the shares submitted.  As the minimum difficulty threshold set by the pool is now 8, which is a bit high for your hash rate (probably 2-3 would be better), it could be that you are effectively under-performing.  That is, where on a setting of 2 you might submit 6 shares and be credited with 12 (6x2), on the higher setting for the same period you may be submitting only 1 share and being credited with 8, even though your farm (vegetable plot?) is working just as hard.

I also find that at certain times of day the peak loading on the internet can slow everything down, with a noticeable effect on my results.