Why can I not set Safari keyboard shortcuts? Conflicts with secondary, unlisted Back/Forward commands

I use ⌘← and ⌘→ to move between tabs. I want to transfer this setting to Safari.


The default Safari keyboard shortcuts shown in the menu bar list ⌘[ and ⌘] for Back/Forward:


[![Default keyboard shortcuts.][1]][1]


Despite those keyboard shortcuts, Safari *also* seems to use CMD-Left and CMD-Right for Back/Forward.


I can set my desired keyboard shortcuts in System Preferences > Keyboard > Shortcuts:


[![custom keyboard shortcuts set][2]][2]


This now works, partially. If the current tab has no Back/Forward history, the browser will navigate to the next or previous tab.


BUT, if the current tab *does* have Back/Forward history, my custom keyboard shortcuts do not work. Safari instead uses the default (but not listed in the menu bar) Back/Forward commands.


Why do these secondary keyboard shortcuts exist in Safari? Where can they be customized? How can they be removed?


`macOS 10.13.6, Safari 12.0`


[1]: https://i.stack.imgur.com/HKHnW.png

[2]: https://i.stack.imgur.com/SvyZr.png