It's quite confusing. Here is a link to a Superior Threads article:
http://www.superiorthreads.com/educa...-measurements/
For threads measured in wt (which is really a length measurement, according to ST), a higher number means a finer thread.
For threads measured in Tex (which is really a weight measurement, according to ST), a higher number means a thicker thread.
That's about as much as my brain can wrap itself around.