Allowing students to re-read passages introduces a new variable to the assessment that is difficult to control for.  That is, some students choose to re-read the passage over again while other students choose not to re-read the passage.  Allowing students to re-read a passage thus increases the variability of the comprehension sub-test score.

By allowing students to read the passages only once, DORA provides a better indicator of how well students will perform in realistic reading situations.  This gets back to the purpose of DORA, which is to provide diagnostic data for teachers to guide instruction.