It's not that weird when you consider that they get paid bi-weekly.

Every month is guaranteed 2 paychecks for them, so budgeting around 2 paychecks is an easy way to make sure expenses stay lower than income every month. But every once in a while they'll get 3 paychecks in a month because that's just how the bi-weekly schedule lined up.

Since weeks and months have nothing to do with each other, the paychecks (delivered bi-weekly) and expenses (charged monthly) will never have a nice consistent match. So working like you seem to be suggesting would mean budgeting every month based on how many paychecks will arrive that month. While their system allows a consistent budget that can be reused month to month with minimal need for change.

Every month is guaranteed 2 paychecks. So if the budget assumes 2 paychecks worth of income, and one month's worth of expenses, then as long as the budget balances they never have to worry about overspending because they thought they were getting 3 checks when they only got 2. And for bonus points the times when they do get 3 paychecks in a month, that 3rd paycheck is loose money. It isn't assigned to a specific use so it can be given to wherever it'll give the best value. Maybe it all goes to savings, maybe it helps pay of an emergency expense, maybe some of it gets used for a fun treat for themselves.

The important point is that they never need to think about how many paychecks they'll be receiving in a given month. So the budget only needs to be adjusted for large-scale changes like rate of income or cost of living, which are much less predictable at an individual level.