So the formula is something like biweekly pay * 26 / 12 (this is from the budget spreadsheet in the wiki)

I get the logic of it, like you take all the paychecks from the year and divide it by 12, and it averages out how much you would make in a month. What I don't get is that this isn't true to reality. Say you work 20 hours a week at $15 an hour. Every 2 weeks you get a paycheck for 600. That's $1200 a month income. But this formula would spit out $1300. If I use this $1300 for budget calculations, then I'm acting as if I have an extra $100 lying around, which could screw things up. And of course the more money you make the bigger this discrepancy will be.

Can someone explain this to me? What's a solution to this? Is there a good reason to be using $1300 vs the "real" $1200 in a budget?

Edit: I also notice a similar thing when calculating weekly spendable money. I must not know something about budgeting and pay periods and all of that, because this doesn't really make sense to me