I have C Band television (the large satellite dish) as well as Dish Network. While having the Sci Fi channel on both, I noticed that there are sometimes different commercials on C Band and DN. Is different advertising sold for different carriers?
Yes. Cable and satellite providers regularly preempt cable channels’ commercials to air their own. In the case of cable, those can be super-cheesy ads for local businesses, or regular national campaigns that buy extra time on the cable network, or promos for the cable company’s own stuff. On the satellite networks, it’s restricted to promos and national ad campaigns for obvious reasons.
On C-Band, you’re getting the raw feed directly from the SciFi channel, so you don’t get any ad preemptions.
Darn, friedo, you beat me to it. 
I have recently learned all about this because Himself now works for the division at one of the local cable companies which sells & inserts the commercials. He’s in IT support, maintaining the servers that store the commercials for insertion. It’s been a very educational experience, and I find myself watching commercials more than usual.