I think (and I can't believe I've read the description enough to think I actually understand it) the first season consists of player seasons 1885-1889. Season 2 consists of those same 5 years, plus the next five (1885-1894). Each season adds the next 5 years, so that by the last season, players from 1885-2010 are eligible. It looks like when you draft a player, you can keep using him over and over again, with the additional 5 years expanding your options for your drafted players.
Of course, it could be a single season teammate twist that I'm just misunderstaning...