Average Days to Pay is calculated strictly on the number of days an invoice is outstanding, without regard to the size of the invoice or the payment terms. It is calculated from the beginning of time. While there has been some lively discussion about how that number should be calculated, there has been no specific consensus.

If you enable the Edit of Customer Statistics, on the System Options tab of the Company profile, you can zero the invoice count and days to pay fields through direct edit or import.

You should be coming to the Adagio Opportunity Conference and lobby for change in this area!