Because of Math

I fill my car with premium. It's more expensive, but less so now than when i was in High School. My senior year gas was $.82, $.92, $1.02 for low, mid and high grade, respectively.

So it was a 24% increase from low to high grade.

A decade later gas prices a have more than tripled. But the difference between low and high grade has remained the same. Now it's $3.05, $3.15, $3.25 again for low, mid and high grade, respectively. But now there's only a 6.5% increase from the low to high.

You should still put in whatever your manual suggests.