By default, cards will be 'snapped' to the scale in the Gantt chart as this makes it easier to plan cards in the planning phase and many users schedule cards by day.
Snapping to scale is implemented based on the following logic:
When updating the start date only, the start date will move to the nearest available start of the day based on the set work hours.
When updating the end date only, the end date will move to the nearest available end of the day based on the set work hours.
When updating both start and end date, the card will be moved to the first available start date.
You can decide to plan on a finer scale and disable this setting, for this open the settings menu and disable 'Snap cards to scale'.
If you like to keep save these settings, save them as a bookmark.