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.

If you want to avoid this bad luck, and get exact expected shares - set diff to 128. That is the lowest miner diff we support for SHA256. But also keep in mind that this will NOT increase your chance to find bitcoin block.