I am curious as to why you did not combine the Roundup and Dual in one trip?

As to your question, it is my understanding that Dual has a bit of "reach back", so that if some grasses are starting to emerge, the Dual can still take them out.  I would not bank on this effect, but my understanding is that it can happen.

I was in a field this week where 3 pint/acre "generic Prowl" was applied.  Looking at the amount of foxtail in the field, you would not know it.  Guy said it was loaded with grass last year.  In this case, I suspect the seed bank just overloaded the system.  That might be what happened in your case as well - plenty of seed in the soil, and not high enough Dual rate for all the seed.