No, it is not normal behavior. The first column is indeed the entity number (sometimes referred to as "slot"; the same entity number used in plugins for most user-specific functions) and will not change for a player that hasn't left the server regardless of if people with a lower id leave.
When "Player 2" leaves, every player above slot/id 2 will remain with the same slot/id that they had before "Player 2" left. I.e. it is common to see skipped numbers in that first column when people leave. The next person to join will get the lowest available slot. So, if "Player 2" leaves and comes back before someone else joins, he will get slot 2 again. If "Player 2" leaves and "Player 12" joins then "Player 12" will get slot 2.
If this is indeed happening like you say then you likely have a corrupt/illegitimate version of the server. Make sure that you download your server via SteamCMD only and don't add anything that tries to bypass Steam authentication.
The only exception that I can think of is if Valve changed it for Counter-Strike only because I verified with Day of Defeat but that is extremely unlikely since they are both based on the Half-Life engine.