That's not true. 500TH/28000TH*6*24*25=64.2BTC/day, or ~2.5 blocks/day.
So if we do 3 blocks/day we do better than the expected:
02-20 1
02-19 2
02-18 1
02-17 2
02-16 5
02-15 2
02-14 3
02-13 2
On average 18/8=2.25block/day.
Two days were better than expected, so your statement of consistently below the expected is wrong.