Some prerequisite information first:
The recipe for garam masala varies across cultures, regions, cities, and chefs. It means "warming spice", so it always contains some cardamom and cinnamon, but the other spices (and the number of them) can vary.
It's used in a large variety of recipes, not just meat dishes.
In many recipes published for an English-speaking audience, garam masala is included as a substitute for other masalas (spice mixes) that would normally be used in the recipe's place of origin.*
With that background:
The reason for "the difference" is flavor. Fried spices taste different from dry-roasted spices and different from raw spices. Spices that you add at the beginning of cooking taste different in the dish than they do if you add them just before serving. So each recipe is getting a specific flavor out of the garam masala. This particularly makes sense if you consider that the garam masala may be a substitution for some other masala.
This isn't hard to understand if you consider garlic instead of garam masala. Sometimes garlic is added whole, or minced or crushed or pureed. Sometimes it's added at the beginning of cooking, sometimes in the middle, and sometimes at the end, raw for a fierce bite. I even have a pasta recipe that involves all three fried, grated and cooked, and raw slivered garlic.
(* when I lived in Kathmandu, our local masala seller had 22 different mixes he offered. Garam was not one of them.)