It has to be 60 days from the end of the calendar month in which the sale was recorded.
If you sold a book on the 31st August, you would receive payment on 29th October (payments tend to go into the bank on the 29th).
If you sold another book on 1st September, you would receive payment on 29th November.
Your sales possibly spanned two calendar months.
Alternatively, if you're talking about paperback sales - the $37+ is the total price, and you haven't deducted the printing costs when calculating how much royalty you expect to receive.