Well, I am not a chemical myself but the question is really chemical.
The carbon atoms can each be connected with 4 hydrogen or fluor or other carbon atoms and the various combinations gives the different HFC with the different characteristics we know:
Attachment 14625
If in two of the carbon atoms we only "occupy" two connections with hydrogen or fluor, the other two will be linked with another carbon atom being "unsaturated".
Attachment 14626
These connections are "weak" and can easily be destroyed, for instance if there is a leak and the gas goes to the atmosphere and receives the sunlight (ultraviolet). In this situation the resulting substances will have a lower global warming potential and so it's a kind of "biodegradable".
That's why they are said to have a lower GWP.
Regards