How many other times were mid-majors undefeated, and how did they fare? Was there a reason BYU was given that benefit of the doubt at the time?
I think it was two things and in both of them BYU was just VERY lucky:
- They got a HUGE boost from playing a highly ranked "Power Conference" type opponent early in the season. As it turned out, they were even luckier because that highly ranked opponent wasn't any good so they basically got a freebie. As I noted above, that win over "#3" moved them up to #13.
- There was just plain Armageddon down the stretch. As late as November 6 BYU was ranked #4 (even that was way too high, IMHO)*. Then #1 Washington lost to USC which moved them up to #3. Then #1 Nebraska lost to OU and #2 USCe lost to Navy in the same week which moved them up to #1. Then #3 OkSU lost to OU. Then in the bowls #2 OU lost to Washington, #3 UF was ineligible.
I strongly disagree with your assessment that they were a top 5-6 type team:
They had two wins over teams that finished above .500 and those were by five points each. Those two teams (Hawaii and Air Force) lost worse to Fullerton State, Colorado State, Iowa, Wyoming, Utah, and Army.
They had another two wins over teams that finished exactly .500 and those wins were by three and seven points. Those two teams (Wyoming and Michigan) lost worse to Nebraska, Oregon State, SDSU, UTEP, Washington, Michigan State, Iowa, and Ohio State.
The most direct comparisons to other teams that finished ranked are these:
BYU beat Pitt by six, 20-14. Against other teams that finished ranked Pitt:
- Lost to #6 Oklahoma by 32
- Lost to #11 USCe by 24
- Lost to #18 Miami, FL by 20
BYU beat Hawaii by five, 18-13. Against other teams that finished ranked Hawaii:
BYU beat Air Force by five, 30-25. Against other teams that finished ranked Air Force:
- n/a, they didn't play any
BYU beat Wyoming by three, 41-38. Against other teams that finished ranked Wyoming:
- Lost to #4 Nebraska by 35
BYU beat Michigan by seven, 24-17. Against other teams that finished ranked Michigan:
- Lost to #2 Washington by nine but note that this was before Michigan's QB (Harbaugh) was lost for the season.
- Beat #18 Miami, FL by eight but note that this was before Michigan's QB was lost for the season.
- Lost to #13 Ohio State by 15.
- Lost to #16 Iowa by 26.
That BYU team was obviously not as good as #2 Washington and #4 Nebraska but they were also not as good as #11 USCe, nor #13 Ohio State, nor #16 Iowa. They were arguably as good as #18 Mimia, FL. A reasonable ranking for them would be somewhere around #17-20 if ranked at all.