It's because of the training. The position a player is listed at by BB should be considered as a "suggested" position based on the player's skills. What happens is that when the skills change each week (even sublevels), the player's salary for each of the five positions is calculated using the position's specific salary formula. Whatever position ends up having the highest salary is listed as the player's position for that week. So often players who may be on the border between two positions may flip back and forth from week to week depending on how they're being trained, and this is perfectly normal.