This is one of the trickier concepts of the ASIA exam. Because there is no myotome testing for the trunk, we have to make assumptions about this region – and the ASIA is standardized to make conservative assumptions. In regions with no formal muscle test (T2-L1), motor function is presumed to be the same as sensory function if testable motor function above that level is also normal. So, even if motor scores are normal in all testable regions, if sensation is impaired between T2 and L1, the motor level is the lowest level with intact sensation. This is also outlined in the Steps to Classification section on page 2 of the PDFs.
If sensation were normal between T2 and L1 for this patient, you are correct that the motor levels would be L4 on the right and L3 on the left.
Let me know if I can provide any further clarification.