Just checking details on IMdB I came across this note about the film Arthur Christmas
Grandsanta says that he is 136 years old. But if he were 136 years old, he would not have been able to be Santa for 70 Christmases and then see his son, the current Santa, complete 70 Christmases as Santa. However, as mentioned in the story, Santas traditionally take their sons with them as young boys. So it would definitely be possible for Grandsanta and Santa to have both made 70 trips since some of those missions would have included both father and son.
In 1951 Edwin R Thiele produced his Mysterious Numbers of the Hebrew Kings dealing with similar questions with regard to dates given for Hebrew kings in Scripture. His solution was similar to this one. Obviously the example is a film and the writers could simply have made a mistake but it may rather be the explanation given. What is true of that fantasy is probably true in a similar way in Israel's history.