This is a really minor issue,
On the page https://docs.katalon.com/display/KD/Release+Notes
Version 5.3.1 seems to have been given the same priority as 5.4, 5.3, 5.2 etc, but none of the other x.y.z releases have been, so I suspect this is just a bug in the documentation, unless there was a reason for doing this? If so it’s not clear.