We are told the connection proces is round robin style so why would this happen?
It happens because it's round robin rather than least used. Remember that over time people will disconnect and reconnect. Lots of people with USB modems will just connect in an evening for a few hours. So imagine that the following happens.
We start off with the platform in good balance and then a large number of customers are disconnected from one gateway. If that happens early in the day or overnight (off peak) then the proportion of customers that just connect for a few hours a day rather than being connected all the time will be lower than if it was at peak time.
Because most of the disconnected people have routers they will reconnect straight away but rather than being put back on the gateway they just came from they reconnect across all four gateways.
This then puts the network out of balance because there are more people connected on three of the gateways than the fourth. It also means that when the people that just start connecting in an evening connect most of them are hitting the gateway where everyone disconnected from because a lot of the tunnels on the other three will be full.
You then get a two-fold of problems. Because some of the tunnels on the three gateways are full, some these, where there are a lot of active customers, will see gold drops whereas other tunnels perhaps won't because they are full of idle customers (and active customers can't connect to these "idle" tunnels) whereas most of the people on the other gateway are all active so again some of these tunnels will see gold drops because of the idle/active balance.
Also, if lots of people are dropped from ptn-ag2, and then reconnetced randomly to the other gateways the on average the number of people connected to ptn-ag2 has lowered meaning lower contention on that gateway. Or ... PN was out of available sessions?
Remember that you can't measure the gateway as a whole, but as twelve 52Mbps tunnels. Only one of those tunnels needs to be full of active customers for traffic to drop and if it's full of enough customers using gold traffic then you may see gold drops on this tunnel. There may not be any drops on the rest of the tunnels on that 622, but gold drops will still show up on the graph because of that one tunnel.
