This might be an old question but I still stumbled on it and figured others would too, so it's still worth answering.
The grind makes all the difference.
Pre-made masa for tortillas doesn't have lard mixed into it. It's just finely ground and mixed with water to make the dough. Great for tortillas.
Pre-made masa for tamales is very light and fluffy because whipped lard is added to a coarse ground masa. It may also have spices added to it, depending on who made it. I get it from a mexican market and they each have their own recipe.
If you make a tamale with masa for tortillas, it will be much denser/heavier, and not in a good way. The taste will be off because lard gives a lot of flavour.
The other trick is adding broth or some of the sauce you'll use to stuff the tamale to the masa dough and mix it in well. It just gives that extra oomph. You don't have to.
How to altar it if you buy the wrong one:
I just ask the employees if the unmarked bag is "masa preparada para tamales", and I never have to alter. If you end up with the wrong type, it may be the wrong coarseness but the taste may be greatly improved if you add about 1 cup of heavily whipped pork lard to 1 1/2 lbs of prepared masa plain masa dough. Add in your spices and other flavourings if you like but the basic is just adding the lard. You have to whip it until it turns lighter in colour, then mix it into the dough. Normally, you don't mix lard into dough, but this is the only thing I can think of to get that ingredient back in.