This reply is based on an educated guess and may not be correct.
I believe the icon you refer to may be a legacy left over from the old ZwiftPower system before it was acquired by Zwift. It used to be a method by which you could open up a rider’s Zwift race/ride Activity and view their power, cadence & HR, as you see from the message with mouse hover. Clicking on the green bar chart now appears to take you to Zwift.com but doesn’t take you to the racers activity.
You don’t see many of these green bar charts. I believe now they signify that a racer’s LIVE information is missing or incorrect. On a first pass of the ZwiftPower results those racers with no or incorrect live data are missed from the results. However this data gets corrected when ZwiftPower fetches a .fit file (blue bolt) which can take some time depending on server load. I believe when ZwiftPower adds somebody to the final .fit file based results it adds this green bar chart to identify the race results change.
I believe this is why you originally thought you were 3rd in the race but eventually ended up 4th.