There is no official meaning for "redshirt" - the way the word is usually used, it means someone on athletic scholarship who didn't get any playing time, so has another year of eligibility. Those students are, however, on the roster even as freshmen.
There are other students who were not on athletic scholarship, thus technically walk-ons, who are not on the roster, but are told they should come back the next year, essentially to try out again, sometimes in a quasi-official way. If they make the roster the next year, are they listed as "redshirt"?
The other thing I would wonder is, how many freshmen who don't play their freshman year (either on scholarship or not) leave the team and go elsewhere? Because if there are two "redshirt freshmen" on a roster, there might be four others who decided to leave the team.