Well, 5-7 can get you a bowl, so using that logic all tteams with 5 wins should be 100%. The fallacy is that teams with 4 wins last week were given 100%. That's not possible. You can't say 100% that any team won't lose all of their remaining games. It's obviously more likely that I'll win the lottery and then be struck by lightning, but still not 100% absolutely positive that OSU won't lose all of their remaining games.
The website is clearly basing their bowl eligibility model on their (current) projections of a teams odds to win a sufficient number of their remaining games to have won >=50% of their games (counting no more than 1 win against an FCS team). They are also clearly rounding their results off at the tenth of a percent, such that 100.0 doesn't literally equal a certainty, but is instead >99.9% likely.