This is the ban interface us staff members have access to. As you can see, there are two fields for ban reason. Your banner obviously only put a reason in one of those fields (ie. the first one). There is no rule regarding ban reasons, nor do I see a need to make one.
fyi, what happens if a staff member does put a reason in the 'reason to be shown to banned'
side note: lololol
 <&jeremych> why was HAM banned?
 <@The_InHuman> jeremych, let me check
 <@The_InHuman> jeremych, ah, he was advising to people to bypass sv_cheats 1 to use cheatprotected commands