I have always found that I can read aloud quite fluently and expressively while thinking about something completely different. My DS6 recently commented independently that he has noticed this about himself as well. I think that this is not an uncommon phenomenon, so the first thing I would wonder (and ask about) is whether your DD's teacher has also tested her comprehension on passages that she has been allowed to read silently. Also, was your daughter allowed to read (i.e. see) the test questions herself or were they asked of her orally? For some people, it's much easier to process what they've seen than it is to process what they've only heard, and this would obviously confound the comprehension test results for your dd.
There have been a couple of threads here in the past few months that discussed processing speed and slowness. If you run a search on processing speed, you'll pull up some threads that might be relevant.
