Well.......its hard to say they are better, although they are statistically safer. Women have just as many accidents as men, however they tend to be minor prangs. Men on the other hand seem to do the job properly when they crash and as a result cost their insurance company a lot more money.
As stated above, their claims cost less. Its worth mentioning that the gap is closing and it probably won't be too long before we are all paying the same. The other reason is 85% of serious road offences are commited by men, which means all men suffer the consequences in their premium.
A case was taken to the European Commission, however, they decided if they ruled in favour that gender could not be used as a deciding factor then prices would rise for everyone. As a result female drivers will continue to benefit from lower premiums than their male counterparts.