While I speak as to PSA or any other company, these guarantees in general are meaningless.

The reason they are meaningless is that the slabbing company is the sole judge of whether they overgraded. If you remove the chip, coin, or card from the slab to have it graded by another company they will complain that the chip, coin, card could have been damaged or switched after removal.

So their guarantee should read like this.

IF you think we were wrong return the slabbed chip to us. If we decide we overgraded the chip (BTW we won't tell you are grading criteria) then we will buy the chip or pay you the difference in value.