Because this is a separate DHCP object from the one Ethernet is using the DHCP server you find may be different from the one that Ethernet found.
Yes, I came to that conclusion myself too. And it is not the kind of behaviour I want, if I get an IP, I most now for sure what dhcp client it came from. So your first post, is the way to solve the problem.
I'll just have to include the changed ethernet.h within the sketch and that should make it portable, doesn't it?