dispatity in maintenance schedule mileage from dealer's notice to owner's manual on 2008 Hyundai Elantra

My Dealer notified me that my car may be due for the 18,000 mile service, however my Owner's Manual & your site show scheduled maintenance at 15,000 miles & 22,500 miles- what could this disparity be about?

Asked by for the 2008 Hyundai Elantra