Could it be simply because UDP packets may be treated with higher priority by all the middleman machines? UDP is used by IP phones, video conferences, etc.
Because of the use of UDP, I wonder the variation could end up being wider. There are some large outliers on the chart (though that could be an implementation maturity issue). Also I wonder if the routing environment will continue to be the same in terms of prioritization if http/3 becomes more common. There might be some motivation to slow down http/3 to prioritize traditional udp real-time uses.