Hello Paul
I did a bit of experimentation myself and confirmed your lab as well. You’re absolutely right. The whole of the subnet that corresponds to the interface in question does not need to be contained within the network address and wildcard mask used in the EIGRP network command. Just the IP address of the interface. Once that’s done, it is the whole subnet of the interface that is advertised. I made a NetworkLessons note on the topic as well. Thanks for taking the time to test this out!
I hope this has been helpful!
Laz