[44net] Difference between RIPv2 and RIP44

I am curious to understand the specific differences between "standard" RIPv2 and the "RIP44" used by AMPR. I read on the wiki that 44net uses a "modified RIP advertisements". I find that tcpdump can decode these packets, showing the destination nextwor and next hop address. Why is a standard ripd unable to process these? Not complaining - just curious :) Thanks, Aaron, M6PIU