why do companies charge double for xlr cables?

Why do they charge double?
Because they can...

Because they can is a good answer. Although a good XLR with gold pins is only slightly more expensive then some RCA's and considerabley less then the better ones. So this is not a good reason. The extra wire thing....well that's the manufacturers problem for not designing a balanced cable to begin with. Generally the shield is what is referred to here as the third wire, so any twin axial with shield can be used as a single ended cable or balanced. Discovery Cable has never charged one penny more for balanced cables and never will.
And for your second part, yes some electronics are not true balanced (phase correct and phase inverted) even though they have XLR's.

Discovery cable is fully balance as well and they will re terminate any of their cables to xlr for $100 They sound great to boot. Joseph runs a fair business- seemingly rare these days. I recommend him and his cabling!

so its a safe bet that if a cable company charges the same for XLR or RCA , that their XLR isn't really "fully" balanced? (Sry just trying to understand this better)