When mining on autoselect I experienced the same and was earning consistently less then I would expect both on the indicicarion Zergpool was giving and on what I expected to earn.

The last week I mined RVN, so no autoselect, and was earning as expected and on one day even way more. Only thing I noticed was that when Individed the bitcoin per block by the coins per block I earned that the result was always somewhat lower then the current price on exchange.

My conclusion is that earning seam to be honest, but the autoselect and autoexchange is not as profitable as indicated on the pool.