Its an in-what-direction-does-the-system-adress-the-hardware-thing.

Just take a look in /var/log/messages after a reboot - there you can see how the first device found by the kernel is addressed as eth0, next one as eth1 and so on. Its possible, that detection routines change with the kernel- (and therefore driver-) version, and thus your cards may be adressed in another order than with Your older Linux system.

I think You can even affect this by swapping pci cards, if You want another order.