Nick
It certainly doesn't tie up with the data you present. In which case I think the FAA publication is a little misleading as on the previous page it seems to be referring directly to the data as represented in the chart.
"For example, the accident rate decreases by nearly 50 percent once a pilot obtains 100 hours, and continues to decrease until the 1,000 hour level." (Possibly not referring to the data in the chart)
"The data suggest that for the first 500 hours, pilots flying VFR at night should establish higher personal limitations than are required by the regulations... Figure 14-4]"
I also seem to recall a chart from elsewhere with a similar blip - who knows, perhaps this is how the urban myth about TT blips started!