As RR suggested in this comment, I excluded the first two points of the curve because the cell were not growing exponentially at that point, or there were too few of them doing so to trust the measurements.
The new graph (above) shows that the dprA- growth curve has a slightly lower slope, rather than a drastically lower one.
Today I transformed KW20, RecJ-, ExoI-, and RecJ-ExoI- competent cells with MAP7 DNA and selected for novobiocin. Tomorrow, we'll see how well the double mutant transforms.
Also tomorrow, I'll be doing a dprA- survival curve and will also learn about the dprA::spec plasmid that SS made. I'll use this DNA to make triple mutants (RecJ-ExoI-dprA-!) and test their transformation frequencies.