What values do you have for Total Days to Pay and also Total Invoices Paid for your customers? Put these fields onto your View (at least temporarily) to explain why your calculated Average is what it is.

The numbers you posted make it seem to me that a number has maxed out at 65,535 - this is often a maximum integer. The Total Days to Pay is an accumulating field, and every time an invoice is paid, the difference in days between the invoice date and the payment date is added to this value. I suppose if there is a maximum number, it could be reached for very high volume customers.

In general, I agree with Steve's comment that this field is not really of good use. Statistically, there are definitely better ways of dealing with this information, especially for the Last Year value.

If you want more accurate numbers and are using the latest GridView version 9.0C, you could potentially calculate the Days to Pay directly and in the way you require by creating an Inquiry on the Transaction Matching file. This stores the invoices and the dates that they were paid. Then use the new formulas to GetLinkedValue and bring those values into the Customer view. This will likely be very complicated (as the Invoice date is not stored in the Matching file, just the Payment date), so if you choose to attempt this, be aware that you will need to combine the 'Customer + Transactions' composite table along with the Matching file, and this is not a simple workspace to create and requires more sophisticated GridView knowledge.