Why do two websocket messages seemingly show up at the same time?

I have multiple servers in multiple locations. I need to get the frequency at which the websocket messages are received. When I connect to a low-latency server, the frequency is normal (50 - 60 ms). But on high latency servers, the frequency sometimes is 0. I asked a similar question not to long ago, but the answer there was that the socket is buffering messages. I find this unlikely since it only happens on high latency servers.

Here is the code responsible for handling the websocket:

        startTime = Date.now();
            ws.onmessage = function (evt)
                {

                prevData = recivedData;
                var receivedMsg = evt.data;
                recivedData = JSON.parse(receivedMsg);

                const endTime = Date.now();
                ms = endTime - startTime;
                startTime = endTime;

                if(msAvg == null){
                msAvg = ms; 
                }
                msAvg = Math.round(((msAvg * 5) + ms) / 6);
                id = recivedData.id;
                }

ms is the frequency in milliseconds.

How can I get to the bottom of this issue?