Hi Gareth,
The current hair distribution algorithm works by the process of elimination. That is, it uses the probability from the distribution map to randomly ommit a certain percentage of hairs. Thus, if you use the map, it will end up planting less hairs/guides than specified. And if the map ommits a lot of hairs then that number will be even lower. If it isn't a huge problem, bump up the number of guides until you get the appropriate count, or make the distribution map blacker.
Cheers
Marsel Khadiyev (Software Developer, EPHERE Inc.)