Probably different locations, one might be in what they consider higher risk than the other. As I said, there's a number of factors that determine the premium.
It's not one price fits all, fee changes depending on amount of rent and a few other factors. The higher the rent the more you pay (just like car insurance - the more valuable the car, the higher the premium).
It depends where your property is and how good your agent is. Mine are blue chip areas and never had an issue with tenants (touch wood). I estimate I've saved well over $5000 in premiums.