Indeed these kits have been around for a good while. I had a go at building a few and I nearly lost heart with kit building as I could not, however carefully I assembled them, get the sides square. Then someone pointed out that the floor was in fact a couple of centimetres wider than the ends!
Trimming the floor helped!
I am building the Ratio LMS 4 plank wagon at the moment and the floor in that is longer than the sides!
I suppose they just get the ratios wrong.
Cheers,
A fellow struggler,
Colin