It looks like he never played for the other team, whose first game was just before you bought him (assuming we're talking about the Finn).
Were the new salaries based on the best position, or the actual skills? It might make sense to calculate a salary based on each position's skill set and then give the player the highest salary. So if your player had a reasonable JS for a C (say mediocre), it might have increased his salary.
Or maybe there is some difference between the way the salary for a new player is calculated, and how it is calculated at an update.