Different numbers for different shifts, days or whenever the casino wants to change so someone can't bring in their own loaded or otherwise altered dice.

Some have hidden numbers under the spots, there are diferent spots, and the logos can have minor variations. Also the size of the dice and the colors of the dice, frosted, or other minor changes, so the cheaters don't have an easy job of stealing from other players and the casino.

Someone found a die with a UV spot on it (not all the spots) so there's one more way.

But the obvious and to answer your question, the numbers are changed so the dealers can see that the same dice are on the table, that are supposed to be in play.