It's all clear about variance.
The weird thing is incremented "lastpayment" entry in the list without real payment occured. I believe updated "lastpayment" (from ./dash-cli masternodelist full, I observe it from 3 of mine vps) means that consensus decided some mn must be paid - and there're no ways to avoid that since the enforcement is active. Correct me if I'm wrong.
It's not about bugs in the monitoring scripts. Looks like dashninja.pl calculates "Last Paid=1d8h37m20s" based on "masternodelist full" output and dashwhale shows "Lastpay=6 days ago" based on the real payment transactions in the blockchain.
Anyway I'll inform about next payment.

Yes, that's correct! Dashwhale lastpay status is derived from the blockchain, nextpay is derived from dashd. So your node moved from the 10% of masternodes, which will be paid within a few hours to the bottom of the payment queue without getting paid and without having been offline, which might indicate a rare bug. Since it's so rare, i suppose it's more worth to dedicate the core team's devpower to Evolution.