Experience orbs get removed by the client (after the client has spent 5 minutes around the orb) when they aren't removed by the server.

Thus, they will appear in /say @e but not on the client, such as in the entity count in the debug menu.

How to reproduce:

Through lazy chunks, where entities do not tick on the server.
Shown here: https://youtu.be/npMqJtaAJyQ 

By using command blocks to modify the age of the experience orbs so they will not despawn. 
Shown here: https://youtu.be/uzdg9frAWvI

By using experience merging, as that modifies the age of the orb.
To reproduce, simply make 2 sets of experience orbs a minute or 2 apart, wait 5 minutes from the first set being made, and the number of exp orbs on the client side will be inconsistent with that on the server side. Since newer exp orbs can either not combine with other exp orbs or the ones that combined older exp orbs to themself, the results are less clear to find as this issue.

Although the videos and worlds are from the snapshots, I tested in 21w03a, 20w51a, 1.16.4 and 1.16.5 and got the same results.

Through testing, neither items nor creepers seemed to be affected similarly.

In the test worlds I attached a backup of, I was testing at was 10000, 1, 0 and in the lazy chunk setup, the render distance must be 2.