This is really important because of the work allowances - if you're paid "double" one month, it doesn't even out over the year. Here's an extreme example of what could happen pinched from the MSE forum:
Anne, a single parent, owner occupier, UC claimant, earning £409 each month and paid at the end of the month, has an assessment period of 15th to 14th of following month. So one monthly pay in each assessment period.
Brenda, a single parent, owner occupier, UC claimant, earning £409 each month and paid at the end of the month, has an assessment period of 30th to 29th of following month. Because of the dates her employer pays and reports her salary she may have say 6 months with 2 pay packets in the assessment period for UC purposes and 6 pay packets with none.
Anne's pay each month comes within her work allowance so she gets the maximum UC award throughout the year.
Brenda's pay only uses her work allowance for 6 months of the year. For the remaining 6 months, 409 x 0.63 = £257.67 is deducted from her maximum UC award. So in total she may be paid £1,546 (= 257.67*6) less UC than Anne over the year.