Default connections and connection<->interface association

[Евгений Шацкий <eugene shatsky net>]

I'm new to NetworkManager.
Compiled and started networkmanager-0.8.4.0, got 2 connections automatically: "System (eth0)" and "System (eth1)".
Somehow they got settings from /etc/conf.d/net (Gentoo config which isn't even supposed to be used, because it's for OpenRC net init script and I'm using systemd instead); moreover, it looks like they got associated with wrong interfaces - "System (eth1)" assigns real eth0 a static address which is specified for eth1 in /etc/conf.d/net, and "System (eth0)" is inactive (no cable plugged in real eth1). After I created new connection and started it, "System (eth1)" went down and dhcpcd started on eth0.
What does this connection!=interface concept mean? How does a connection get associated with an interface? Couldn't see anything relevant neither in both default nor manually created connections settings... Console "nmcli con status" shows this, but only for established connections.
How default connections are created?
How did systemd read settings from /etc/conf.d/net? Can's see anything Gentoo-specific in ebuild except --with-distro=gentoo configure option. Running NM with "--log-level=DEBUG --log-domains=HW,RFKILL,ETHER,WIFI,BT,MB,DHCP4,DHCP6,PPP,WIFI_SCAN,IP4,IP6,AUTOIP4,DNS,VPN,SHARING,SUPPLICANT,USER_SET,SYS_SET,SUSPEND,CORE,DEVICE,OLPC" didn't show any clarifying details.