Yes, that seems reasonable. There are four biologically possible scenarios I can think of to explain the numbers:

1. It's developmental noise.
2. Mutations that cause hermaphroditism arise at a certain rate and are eliminated by natural (or artificial) selection at a certain rate; this is mutation-selection balance.
3. Multiple genes at different loci are required to produce a hermaphrodite (this is epistasis); natural selection doesn't act against these genes since it is rare for them to be found in the same invididual, and they may produce some benefit when apart.
4. Hermaphrodites have reasonable fitness and are held at an equilibrium frequency in the population.

Four seems far and away the least likely; I'd be suspicious of an equilibrium that's so low, not only in our species but all our mammalian relatives. Perhaps there are answers in the literature; I don't have the time.