Smaller grits are significantly coarser than specified. More like 220, not 800. The two medium grits ("240-320β and "400β) appear to be exactly the same grit, even under a microscope. Return them and choose 3M instead which I can trust. Edit 6/11/19: It turns out that the 3M 400 grit ('extra-fine') sponges aren't all that different from these 3M 400 grit sponges and these are a bit rougher than 3M 400 grit sandpaper, that's more equivalent to the 500-600 grit sponges in this set. So I decided to keep her to myself. However, sanding 3M plates works better for me as I don't sand perfectly flat materials.
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