devtrackers.gg
That's my assumption too. Look back some cards have the extra bit, some don't. I'm guessing blizzard add it whenever they can, and ignore it otherwise.
whenever we make multiple cards that generate tokens with pretty unique/long effects, we try to have at least 1 card whose focus is explaining the mechanic rather than doing something more complex/unique with it. in this case, Explodineer is a simpler design that explains the entire thing, letting us do a bit more with Safety Expert's design!
some other examples:
In Nathria, we made Volatile Skeleton a collectible card. because it's collectible (and common, so its more likely for players to own it), it's more likely for players to be able to search for it, and therefore know what they are in the context of cards like Cold Case!
Remember Tamsin's hero card from AV? neither she nor her hero power say what a "Rift" is... but Seeds of Destruction does! Tamsin and her Hero Power have a lot more going on than just shuffling rifts, but Seeds of Destruction's entire effect is JUST shuffling, leaving plenty of room to explain.