Every company is different, but leasing agent commissions are often split with the entire front office team because each person plays a part in the process of taking that prospect to resident status. Making sure the apartment is ready, the screening is approved, the lease is properly prepared, the payment arrangements are handled, etc.
While I agree it should have been more clearly stated in the offer letter, splits allow for minimization of the drama that can happen in an office when people are territorial about commissions. Splits should result in better customer experiences because everyone wants that prospect to become a resident, so everyone is willing to help make that happen - even if you're at lunch when they come in to sign their lease.
Splits are tough if someone is not doing their part, so a great Community Manager will make sure that all team members contribute in the leasing process. Hope that is the case where you are working!