Call Routing to Secondary Groups

Curious if anyone else has experienced this:

We have our phone teams set up in groups, a primary for specific products and then an overflow group that contains everyone.

In addition to seeing weridness WITHIN a group of calls more heavily going to the same resource even if they just got off a call and others are available is weirdness BETWEEN groups.

It's my understanding for Talk routing that when a call comes in the system looks for an available agent and picks the one with the most time since their last answered call. I'm not sure that's working within a group but I am curious what the expected result would be with multiple groups.

For instance:

Conditions:

Group 1 contains: Rep A, Rep B, Rep C
Group 2 contains: Rep D, Rep E, Rep F
Group 3 contains: Rep D, Rep G and rep H

Calls come into Group 1 as primary and then Group 2 resources are secondary.

When a call comes into Group 1 it's serviced by Group 1 or goes to Group 3 as overflow

When a call comes into Group 2 it's serviced by Group 2 or goes to Group 3 as overflow

Scenario 1:

Call comes in at 8AM to Group 1 and Rep A is available and gets the call. Rep A finishes the call at 8:10AM
Next call comes into Group 1 at 8:15. All reps available. Expected result is that Rep B or C would get this call because A just finished a call and would have the shortest time since last answered call (unless the system is treating the fact that B& C haven't answered any calls as 0 time since last answered call and therefore pushing them back in the queue). What we routinely see is Rep A getting that call while B & C remain idle which should only make sense if the other two had shorter last answer times (which again COULD be the case if they haven't gotten ANY calls yet and the system is reading that as 0).

Scenario 2:

3 calls come in simultaneously to Group 1 at 8AM and all three reps (A, B and C) are occupied.

3 calls come in simultaneously to Group 2 at 8AM and all three reps (D, E and F) are occupied

At 8:15 Rep D finishes their call.

A call comes into Group 1. All those reps are still occupied so call then goes to secondary group (Group 3). In this case, since D just finished a call, Expected Result G or H would get a call. Actual result D routinely gets this call. 

Does the system not track the just finished call from the other group or is it tracking G and H as zero elapsed time since last call and doesn't assign because that's less than D's elapsed time?