Unfortunately, dark coloured slate does show scratches very easily and there are loads of ways they can happen. from left over cutting residue and grit from the factory, the way they've been handled, transport damage, the list goes on. Regardless of who the supplier is or the country of origin, you never get 100% perfect slate. But its a natural product so you shouldn't expect it to be.
Clear sealers can disguise them a little, but the only sealer I've used that will make the scratches completely invisible is LTP Stone Oil which I tested on honed slate.... you can sneeze on that stuff and it scratches! (other similar oil based sealers are available by the way). Minor surface scratches are inevitable even after fitted, grit on shoes, dropping stuff on them etc.... the stone oil will colour the scratch to the true slate colour, blending it in perfectly. All you have to do is dab a bit on a cloth and rub it into the affected area and buff clean. The whole scratch repair process can take less than a minute although full curing of the oil will take longer. Deeper scratches may need sanding down before applying the sealer.
So use Stone Oil as a primary sealer, couple of good coats before grouting and a further coat after and you won't even notice the scratches were even there, and keep some back for doing future scratch covering.
Actually.... Come to think of it, here is a PDF of the test I did. Aimed at honed and brushed slate but works for riven too. Hope you find it useful.
