I can think of several, but the answers may not necessarily be correct.
The least-connections is usually correct in its processing. However it's least CONCURRENT connections at the time of the LB selection. It's possible that the :443 and :80 connections take different amount of time to complete a connection, which could be skewing the results between them.
Is it also possible you're selecting by node for one and member for the other? Or one of the nodes has additional connections from another pool (And you're selecting node and not member least connections)
H