There is a difference between speed being a 'contributing factor' and speed being 'the main cause'. The five percent sounds like the statistic where speed is the main cause, while the one-third sounds more where speed is a contributing factor.
Anyway, I have a hard time believing speed statistics from government sources. For example, when the National Maximum Speed Limit was established (which, by the way, was a violation of the 10th amendment), advocates crowed when there were less traffic fatalities the following year. However, when deeper investigation was done, it turned out that the energy crisis of that time caused less miles to be driven and it turns out that Americans driving less miles, NOT the reduced speed, was the major factor in the reduction of traffic fatalities.