It's pretty simple why it's a fucking stupid way of doing things, really.
Say somebody puts a claim in on the 14th of the month. After the waiting for the ID appointment, the actual claim appointment and the ridiculous length of time for the claim to be set up, they find that the assessment period is the 24th to the 23rd of each month (for example). Their salary goes in on the 24th.
Seems fine. Payments start turning up. Eventually. Two months after the claim was submitted.
BUT
They haven't managed to get the notification of earnings through, despite claiming that it is done automatically through the tax office by computer in real time.
By month 3, suddenly the payment screen says they're getting nothing in five days. They try and ask why. They're told 'read what it says on the screen'. Which says their earnings are three times higher than they actually are. Crap. No money this month. This is a bugger for paying childcare and rent in advance, never mind the rent/mortgage or getting prescription charges covered (the inevitable penalty fares for not ticking the box that doesn't exist on the prescription form - the one that should say 'is in receipt of Universal Credit' - come through. Hundred and twenty quid a time. You can appeal and they'll cancel it - if you know that they've got it wrong and you are entitled and not terrified into paying it by the threat of criminal charges and thousands of pounds of fines for not doing so).
The next month, the amount actually paid is a lot less. But at least it's something, right? Even if it's a five week month, it's still something.
Then, because the 24th of month 5 falls on a Sunday, payroll pays salaries on the Friday 22nd.
Month 6, log in and they're getting £0 because they've earned 'double' this month. It's a five week month. This is bloody tough.
Month 7, log in and your claim hasn't been updated at all. Two days later, you have to confirm a few things (that you've read the bit about not committing fraud, largely) and wait.
Three days later, you find out that your claim has been cancelled and backdatedly cancelled to two months ago, so you have to reclaim. From scratch. Taking into account that you're automatically locked out of the details of your previous claim, so can't access a single details. Three weeks later, you've finally got your proving your identity appointment, your boss is pissed off that you've taken time off (a whole afternoon) and you've still not received anything. It's Christmas, so payroll stick your salary in on the 21st.
So you get nothing again because you've had double salary in the 24th - 23rd December assessment period.
You might get something for January. Might. You'll find out after payday.
It is utter shit. And somebody was paid an absolute fuckton to come up with this method of calculating claims.