I do believe the reason is there is not enough masternodes to soak up each block.
They need to wait atleast 102 blocks for the previous mined block to mature before new block, i GUESS.
+more.
But MN, that I mentioned, didn't receive any payments yet
https://solaris.blockexplorer.pro/address/ShbCA6NEZAVMGfqb2WdxzfPRQdbrMrh4gm So, there was no reason to not pay to it. Or there is some time period of masternode maturity?
Interestingly it seems to be around 3h26m between my rewards.