Is it normal to have positive ulnar variance after surgery. other words to have metal plate inserted in wrist that is too short?

No: Positive ulnar variant refers to the fact that the ulna extends beyond the articular surface of the radius. If you had radial fracture would indicate compaction of the fracture resulting in shortening of the radius. If it was an ulnar fracture then may be distraction at the reduction site lengthening the ulna. This can occur as as a natural growth abnormality diagnosed by previous xrays.

It depends: On what was your ulna variance prior to the original surgery. Many times it can be helpful to look at the other wrist. This is a common finding after a distal radius fracture as there can be some impaction and shortening. Also, this can be a normal variant in some patients.