Maryland resident; univ Delaware student. I worked p/t at UD and earned $750 W2. Filing non-resident and it's telling me I owe $4.00? Why do i owe?

TurboTax's computations on your Delaware return are based on the state's tax law, and on the information that you entered into the program. 

To see how your Delaware return was computed in TurboTax Online:

1. Open your return in TurboTax
2. Select Continue or Pick up where you left off
3. In the left pane, select the Tax Tools dropdown
4. Select Tools
5. Select View Tax Summary
6. In the left pane, select DE Tax Summary

Note that, according to Who is a Resident? in Delaware's Individual Income Tax Return Instructions:

Full-Time Students with a legal residence in another state remain legal residents of that state unless they exhibit intentions to make Delaware their permanent residence.