I agree that SEGS and especially PCI are better than most.
I think in general, the lower the grade, the better the chance of it crossing over with a very similar grade. I bet F-15s will tend to grade very close when crossed-over; the MS-65s, not so much; the MS-70s, forget about it.
However, there are some "basement graders" that truly are in a fantasy world. The above does not apply for these. I think they just print up labels, then find random coins that fit in the holder.