There are mainly two reasons for that:
1. Some items in your order are back ordered ones. We do not want to delay the delivery of the remains. So items in stock will usually be shipped in advance. Back ordered items will be shipped out timely once they are sourced later.

2. Some special items which include easily-detained accessories, such as batteries, will be shipped separately by the carrier, especially when you choose express mail like DHL/UPS/Fedex.The main product(s) will be sent by the shipping method which you chose while the accessories like batteries will be usually sent by regular post service, which could be cleared by the customs easier.