It is called bad luck. Miners have to be "lucky" to find shares. And higher the diff, the more luck is needed. I think someone good with statistics can actually calculate you the odds to have 20% difference.
For the probability of that ocurring, since you mentioned that, someone corrects me if i'm wrong, each share is independent and has an uniform probability of meeting the difficulty set, so we are talking about a binomial distribution with the following parameters:
N = number of produced shares
P = difficulty_of_shares/target_difficulty
K = number of accepted shares
Considering that you only pay for valid shares, and you say they are of difficulty 128, and that my target difficulty was 12000 most of the time, we have:
N = 784869659/128 (since you are paying for shares with difficulty 128)
P = 128/12000
K = 668995673/12000
This number is so small that it i have been unable to calculate it precisely. But i made a simulation with much smaller differences, ranging from 1% to 5% that you can see bellow.
| %actual shares | N | K | Probability |
| 99.00% | 5226528 | 55192.13568 | 0.008762397104 |
| 98.00% | 5226528 | 54634.63936 | 9.52E-07 |
| 97.00% | 5226528 | 54077.14304 | 4.14E-13 |
| 96.00% | 5226528 | 53519.64672 | 0.00E+00 |
| 95.00% | 5226528 | 52962.1504 | 0.00E+00 |
As you can see, with a difference of only 3% the probability is already so small that you should already be suspicious if it occurred, with 4% it's already not representable by a double precision digit.
Now, we are talking of a 20% difference. Please, do not blame luck. Luck was not involved here!