Whenever someone replies to a person within the public activity feed, their level that is displayed is incorrect.
Instead of showing their level, I believe that it shows yours instead.
https://imgur.com/a/fPycefe
This image shows it in action. As you can see, both 1 and the beet have shown levels of 87 in the replies, despite 1 having a much bigger level and the beet having a level of 88. I believe it is whoever is currently seeing the page, as it was level 86 when I was 86 (no picture evidence, sorry), and it is level 87 now that I am 87.