Yeah not sure why some do and some don't, but it's common. My Diamond Professional marks firmly-hit balls. That's after pocket replacement; before they were replaced the balls would look like someone drew on them with a fat black marker.I keep heating about the marks from Diamond pockets but I honestly have never found a mark from the pockets on my Diamond Professional pockets in the 7 years or so that I have had it.
My one leagues Diamond bar boxes that they bought used, used to mark the balls quite a bit. You could scrape them off with your fingernail.
The newer Diamond tables at a place I play at also mark the balls. The near-30 year old Diamond tables at another place I play never mark the balls. Perhaps all the dye has already rubbed off on those.
Ball marking is a disappointing reality for otherwise great tables.