Anderson had the option of going to 170, where GSP would have fought him. I think GSP and Anderson (and Jones) were great fighters, but none of them were willing to change weight to fight a dominant champ. To a certain extent I think Anderson was trolling people when he asked GSP to fight him at 185 -- calling out someone and telling him to fight at your weight is pretty silly. GSP could similarly have called out Anderson to fight at 170, but wasn't into trolling the public the way Anderson was.
Basically both knew that if they fought at 170 then GSP would have won, if they fought at 185 Anderson would have won, and catchweight was only mentioned once, because making it a non-title fight would have meant huge loss of sales to casuals (and both GSP and Anderson got excellent PPV shares).
If you can duck someone not in your weight division, then GSP ducked Anderson at 185, Anderson ducked GSP at 170. But that's a pretty silly definition of duck unless you're advocating getting rid of weight divisions. Which actually I'd like -- none of the combat sports initially had weight divisions, they were only added for marketing reasons. If contact sports like football and hockey, and size related sports like basketball can exist without size divisions then there's no reason combat sports can't either.