Hello, individual levels (IL's) use in-game time (IGT) while other categories use real time (RTA).
I think the main reasons for this are: IGT has been the standard for years I think mostly due to convenience. It's nice to have instantaneous feedback after completing a run. Additionally playing on a 3ds can make it difficult to figure out the exact RTA time of runs and retiming IL's is generally a bit too time consuming for the mods.
The main downside of IGT are some levels having lots of tied times on the easier levels and some levels having different stratergies to fullgame runs because of screen transitions, and powerups e.g., W11-2. However, despite the lack of precision IGT is the standard across the majority of Mario games for IL's.
The Moderators might have a better explanation but that's why IGT is used.
If you're interested there is a list of all the fastest IL's RTA as of 1st of March 2022 put together by Charbunny. https://docs.google.com/spreadsheets/d/1gROleFgqnC5h3auMUw2REGzrJ7Ctse348491ej0l0WA/edit#gid=0 I have outdated a few of them...