But, I think the Big 12 CCG tool is off.

Once KSU is deemed #1, you move to the two-team tie breaker process. For the two-team tie-breaker, the first is head-to-head. The second is record against all common conference opponents. Iowa State and BYU common opponents are:
Baylor, Houston, Kansas, UCF, Utah, and KSU. So, if BYU's loss is @ASU, then BYU is 6-0 against common opponents and ISU is 5-1 against common opponents. Advantage BYU.

If BYU's loss is to Kansas, Houston, or Utah, then both are tied at 5-1 and you proceed to the next tie breaker...which is "Win percentage against the next highest placed common opponent in the standings (based on the record in all games played within the Conference), proceeding through the standings."

"Proceeding through the standings" is key. Looking at the standings, the highest placed common opponent in the standings is clearly Kansas State. BYU has a win; ISU has a loss. Advantage BYU.

It definitely gets more fuzzy if Colorado is also in the mix because you have to go through the multi-team analysis again, but comparing just BYU, ISU, and Colorado (and not KSU).