In TeX mode the procedures backward one word and move one word use an ad hoc definition of word, while the procedures that extend the selection use the standard definition.
Hence there are different behaviors; when moving the cursor "\frac" is a word and "4x" is considered as two different words, while when selcting "\frac" is not a word ("frac" is a word) and "4x" is one word.
It would be better to have coherent behaviors.
This is fixed now.
Changes committed to the repository (rev. 1323 (sourceforge.net)). The core must be rebuilt.