It's a problem with floats.
From Graeme at SweetScape............
"The error you are seeing is caused because 'float' is not very accurate for doing a lot of computations. We tried your function in C++ and it gives the same issue. 'float' is just not very accurate and if you switch to 'double' it will be better, although double has accuracy issues as well for lots of computations."
Then next section (Faces) uses all ints so shouldn't have a problem.
One thing I am doing is writing separate scripts for each part.
Verts, Vertical Textures, Faces are all separate scripts.
Makes catching errors easier.
The 1st script calls the next 2 in order just as if it was all one script.