I recall when I started driving (on my dads car ins), the policy evidently increased, and adding my brother on the same policy dropped it by £7.00 - apparently it was simply that although there were 3 people on the policy, there were still no accidents / claims. Apparently the other factor however is that as a named driver on a policy, the insurer works on the principle that the named driver/s will have use of the vehicle only part of the time, whereas a policy holder is regarded as having full-time use of the vehicle.