TurboTax will use whichever of the 2 forms can most simply report your info. In the desktop version (CD/download) there is a way to override TurboTax's selection and use the full Form 2106 even if one qualifies for the Form 2106-EZ, but not in the Online version.
What's wrong with using a Form 2106-EZ?
See this screen image on the qualifications to use Form 2106-EZ:
