This is very simple, when you deploy a device uisng PXE, ConfigMgr will inventory the MAC address, but that will prevent that mac address from being used once more unless the hardware inventory is executed after the machine has been deployed and removed from the docking station (similar)

The fix:

• Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\Components \SMS_DISCOVERY_DATA_MANAGER on the primary site server
• Add a MultiString entry called ExcludeMACAddress
• Add all Mac Address to ExcludeMACAddress

For a complete story I strongly recommend you to read the following post:

http://blogs.technet.com/b/system_center_configuration_manager_operating_system_deployment_support_blog/archive/2015/08/27/re_2d00_use-the-same-nic-for-multiple-pxe-initiated-deployments.aspx

/mike