Variance.
If donation is a small % of a large amount of GH/s then the payment will be small but constant.
0.16 per block would be roughly 100% of 1.15 GH/s. However the donation is most likely the default for most users that donate. So it isn't 100% of 1.15 GH/s. It is 0.5% of ~200 GH/s. If you had 200 GH/s your payout per block would be very constant too.
