There are two common reasons:
The main reason is due to the nature of the rewards distributing rules on the Aleo network. In each block, the network rewards multiple quantified miners with different weights due to their comprehensive rules considering a series of matrices. The difference in rewards from block to block can be significant for the same miner.
Payout scheme difference is another factor. You may also find that rewards in your dashboard are not generated evenly in different time slots, within a given period of time. Since the PPLNS payout scheme only records rewards after the mining pool actually mined a block. But in the long run, the rewards are almost the same as PPS.