Linux: single NIC is named eth0/eth1 due to udev and MAC-based rules. Kinda weird

I have a motherboard with onboard NIC (single Ethernet NIC in the computer); also USB port is constantly occupied with Wifi dongle.

in my /etc/udev/rules.d/70-persistent-net.rules, found this:

# PCI device 0x10ec:0x8168 (r8169) SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="6c:f0:49:e8:22:cd", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"

# PCI device 0x10ec:0x8168 (r8169) SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:1e:68:dc:96:52", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"

Currently, eth0 is detected. If I reboot, it may become eth1 (don't sure if mac will change, I wont reboot this PC today).

# ifconfig 
eth0      Link encap:Ethernet  HWaddr 00:1e:68:dc:96:52  

Sometimes, eth0 is not detected and eth1 comes up. Why? Can it be some BIOS bug with setting onboard MAC when cold-booted improperly?

Currently the ruleset is changed to force name to eth0 (for both macs), should the next reboots reveal if it helps..