I think you're failing to adjust the stats for how good BYU is/was. Here's a good measure for thought: ESPN FPI game score.
If we look at Utah's FPI game scores against BYU, here's how they look:
2018: 54 — this is the worst example, where 54 is the #10 best game out of 13 that year
2017: 69 — tied for #6 out of 13 games that year
2016: 79 — tied for #4 out of 13 games that year
2015: 88 — #4 out of 13 games that year
2013: 94 — #2 out of 13 games that year
2012: 84 — #1 out of 12 games that year
2011: 92 — #1 out of 13 games that year
You could easily argue that there's a downward trend in how Utah has performed against BYU, but overall Utah is still performing above average against BYU relative to how they play other teams when you factor in BYU's relative strength.