[jchcollins]

Darren, also with your '67 Mays example -

Some may appear to be "swimming" in the PSA slab because they have tilt or a slight diamond cut. Both are incredibly common on '67 Topps. I do buy into your theory however that microscopic differences in size are also very common. Cards that "swim" in a TPG holder for whatever reason are one of my biggest pet peeves. It's just the annoyance of seeing them move more than any fear that they will get damaged that bugs me most, but I will generally liberate a card from any slab if it moves too noticeably. Toploaders and penny sleeves and you don't have that problem...