If I'm picturing it right, it's to do with the category settings.
Have a look at one of the categories you've overspent on in August. Click on the red figure and you'll get a pop up box. For each category you can choose whether the overspend stays in the category or comes off next month's available money.
So, if you want to reduce September's money available, choose "Subtract it from next month's available budget" - that was you'll see the "Overspent in August" figure at the very top of September's column change. If you want to carry the overspend in the category, choose "Subtract it from next month's category balance" - that will mean that it's the category that's 'overdrawn' but it doesn't affect what money you have to 'spend' in September.
Carrying the category like this is useful for when you have a category you know will be topped up - so, for example, my husband and I both claim expenses from work. Our 'work expenses' category always carries a balance over, because we pay out but don't get it back in the same month. You do have to be careful though, because if you are running close to using up the actual money you have in your accounts, carrying too many categories over means you risk running out of actual money as well as budget. On the YNAB forums they call carried categories like this 'monopoly money'.
The other thing that YNAB would tell you is to adjust your budget in some of the other August categories to deal with the overspend and keep it clean. So, for example, if you overspent on 'food' but didn't use all of your 'child clothes' category, the recommendation is to reduce the 'child clothes' budgeted figure, and add it to the 'food' category instead. They would argue that this is a more realistic way of acknowledging what you're actually spending. I do this sometimes, not others, depending on the category.
Wow that got wordy - hope that was what you were actually asking. 