That is a misinterpretation of the sentence on your part... "a well-placed little birdie told me over the weekend that they sold a grand total of 503 Kins before they pulled the plug".
The "over the weekend" phrase is marking when he was told, not when the sales took place. This interpretation is further supported by the "that" which immediately follows it, so as to separate the part of the sentence about the act of telling from the part of the sentence about that which was told. And further supported still by the use of the term "grand total" in the phrasing "grand total of #X before time_Y".
That pedantic enough to prove the point?
Accounting for registrations by unsold units is a more likely source for the difference in numbers.