I don't see how that bit of difference (in the length from ridgeline to edge) could cause the problem. The two sides could be completely different and it should make no difference.
I still think that something happened at the ridgeline when the two halves were sewn together or at the bottom edge when the grosgrain was sewn on.
If the last couple of feet of one side of the ridgeline was stretched as the halves were sewn together, I think it could cause the floppy edge. Also, if the bottom edge were stretched when the grosgrain was sewn on it could cause it.
The problem with my theories is that you would have noticed problems as you were sewing...i.e. one side came out too long as the ridgeline was sewed or the edge was coming out longer than the grosgrain and had to be scrunched back at the end to make them match. Anything like that happen?