I have an eye-clean SI1 stone that has a lot of inclusions, but they're all very small and spread out. So I think the number of inclusion in my case is giving it the rating.
But with the loop, I have a pretty hard time picking them up. At like 30x magnification it's not a problem..but at the usual 10, I really have to look for them....particularly the clouds (A dashed outline on the cert plot -- at least on AGS)...and that's one of the largest inclusions on my diamond...a cloud under the table by the culet.
So I think it may depend on the stone. You might be like me and have an SI1 that's graded as such because of the number, not because anything is going to jump right out at you.
I also have a "needle" on the edge of mine that looks like an icicle at like 30x. Don't know how that might also effect the grading.