Your network interface still has information about your regular internet connection, even when connected through a VPN. Local apps can query this information. Webrtc clients do this in order to assist setting up direct connections -- it doesn't know you're using a VPN, and will happily hand over info about your network interface to any site.

This only happens if you're using windows as administrator and your VPN runs directly on your box. If you're running OpenVPN on your router your real external IP won't leak.