One can come up with reasons as to why the numbers at the Battle of the Trident were what they were, but in comparison to their total manpower, the ratio is still astonishingly low.
After all, if they don’t win the war, they’re all going to die, so that’s a pretty strong argument for putting as many men into the field as possible.
This is one of those things I think people would feel a lot better about if they just accepted the Doylist reasoning; AGOT was written in like 1993 when much of the Seven Kingdoms and the facts about them hadn’t really been conceived of an implemented yet. The numbers we’re given there are simply wrong when considered in light of later resources and should be ignored, in the same way we shouldn’t try and figure out how a lemon tree was growing in Braavos.
I don’t think you can even characterize it as a fuckup on Martin’s part, unless someone wants to make the argument that Martin should have used the Battle of the Trident to determine all subsequent demographic information about the polities and armies involved. I don’t think anyone wants to do that.
If Martin lives long enough, he may release a slightly corrected edition that fixes some of these little errors. Robert Jordan did that, in fact! In one of the early Wheel of Time novels, he gets a line about harp-tuning wrong. Fans continually beat on that point, and in later editions the line was changed.
I would love a corrected edition…especially of the WOIAF.