That is no true. For example, a 10th-level character needs 10,000 xp to become a 11th-level character. And a 11th-level character needs 11,000 xp to level up. Current level by 1,000 XPs. This is the formula.
A cohort gets his level/leader's level by the XPs the leader gets. So, when a 12th-level master gets 12,000 XPs (enough to advance in to a 13th-level character) throughout a certain period, his 10th-level cohort gets 10,000 XPs during that same period, which is enough to push him up to the next (11th) level.
Now, if the said cohort was somehow at lower level, say, 8th, he only gets 8/12 of the XPs his leader gets. So, throughout the above period, he will earn only 8,000 XP. Just enough to push him up to a 9th-level character. This continues. Once the cohort become certain levels behind the leader, unless the leader somehow lose a level or two, the cohort never catches up. The difference in levels between the leader and the cohort never decreases.