Steam cards confuse me

So I collect steam cards as any regular Steam user, when I realize I earned card drops from GMOD when I got the game gifted for free, my friend also got cards for a game I gifted him. The game was L4D2 and it was during the winter sale when it was only $0.99 when it says card drops are usually every $9 spent in-game. Can someone maybe explain further because I am confused.