The odor of some fish oils is due to sulphur. Fish oil can be low or high sulphur depending upon the freshness of the fish and whether the oil has been properly kept cool. I don't know if there's any additional process to lower sulphur content but there doesn't need to be if they simply exercise quality control.
Here's a document from the fisheries and aquaculture department saying these are the factors impacting sulphur content in the production of fish oil:http://www.fao.org/DOCREP/003/X6899E/X6899E11.htm
Where fish burps make it seem as if there's a rotting fish in your gut in a way there is. Personally I've noticed less than fresh orange juice also becomes sulphurous. If it isn't fresh why have it in your body? As for me I'll stick with the burpless variety. More expensive in some cases but well worth it in my opinion.
I get a vision of a fishing boat breaking down and then returning to port with two week old, spoiled, fish. 'No worries', says the Captain,'We'll just make fish oil!'. No thank you. I'll take the fresh stuff please.
The type I take also says it has been purified to eliminate mercury, a big concern with fish these days. I can't see how getting rid of mercury could be a bad idea either.
As noted though flax seed and/or oil seems to be just as good and tasty too. Personally I use both but not everyone is a control freak.