Those are not fractions of a second you are incrementing; they are frame counts. That is a standard timecode format used in video – hours:minutes:seconds:frames. I am fairly certain this is in FAQ but also explained at the bottom of Shotcut - Keyboard Shortcut Reference, the Documentation - English Glossary / Terminology, and numerous posts in the forum (discoverable if you searched here or the web or AI).
Finally, there is Settings > Time Format where you can change it.