Not a rules expert, but this is the wikipedia explanation:
College football
NCAA rules are less permissive than NFL rules and require that the five interior linemen, numbered 50 through 79, never line up as eligible receivers.[10] If an offensive tackle wishes to line up as a tackle-eligible, he must physically change his jersey number to that of an eligible receiver (this, for example, happened in the 2015 Cotton Bowl, when an offensive lineman for Baylor switched from his usual number of 60 to 80 in order to perform the feat).[11] A defensive lineman can line up as a tight end if his number is not between 50 and 79, as defensive players have no positional numbering restrictions in the NCAA. [citation needed]
