This varies by company and location. Most companies begin rental charges at your designated pickup time location. If you say, “Our first pickup is at 6pm at 123 Main St’, then this will be the beginning of your rental charge. Some companies begin rental charges on portal-to-portal basis and charge ‘travel-time’.
Specifically, your rental time may begin the moment the limousine begins its journey to your first pickup location. Ask your limousine company specifically how they charge as this might explain price differences. Charges for travel time is usual in remote areas where the closest limousine service may be many miles away.