It's as simple as percentages
There are 68 NCAA Football teams that play in a bowl game out of 119.
There are 65 teams that play in the NCAA Basketball Tournament out of 347
57.1% of Football teams make a bowl game
18.7% of Basketball teams make the NCAA tournament
So the Gophers Football team barely squeeking into a bowl game probably means they are about 50th in the country. The Basketball similarily were probably about the 50th best in the country but being 50th out of 119 is a lot different then 50th of 347...
No, it's not that simple.
If the 7th place team in the Summit League were as likely to make the tournament as the 7th place team in the Big Ten, your statistics would be true. Since we know that's not true, the supposition is false.
What's far more instructive is to draw from the sample size that actually makes the tournament every year, not set up a false dichotomy by saying that all 347 teams have a legitimate shot of making the tournament. In reality, most conferences are auto-bid only. We are not competing against Bethune-Cookman or South Carolina Upstate to make the tournament. We are competing against Washington St. and Auburn.
There are 31 auto-bids and 34 at-large bids every year. Realistically, 17 conferences will almost never get an at-large bid, meaning their constituent members are not part of the pool for at-large selections. I'm not going to take the time to add them up, but given that there are 32 "units" (conferences plus independents) and 347 teams, that works out to an average of 11 teams per conference, plus the independents. That already takes 181 {[(18 * 11) - 17 auto-bids] = 181} teams out of the equation.
So, in reality, the 166 (roughly; again, I'm not adding them up right now) teams that make up the BCS and mid-major conferences are in contention for the 34 at-large plus the other 14 auto-bids. That equals 48/166 = 28.9%. Statistically speaking, the Gophers enter the season with somewhere around a 30% chance of making the tournament based on their conference affiliation, not 18.7% as you say. (It's actually higher, really, when you factor in that the BCS conferences almost always have more at-larges than the mid-majors.)
For the same reasons, i.e. Florida Atlantic at 6-6 is not as likely as UCLA at 6-6 to make a bowl game, your football numbers aren't correct either. But the point I'm trying to make is that it's not just as simple as dividing potential spots by the overall pool. If it were, there would be no disparity between the larger and smaller conferences, and no incentive for teams to remain in the power conferences.