What you are seeing is actually expected behavior for time entry at this time.
This is because of the way time is stored, it is actually stored in an integer (not decimal) amount of minutes. The UI translates this between a decimal amount of hours to an integer amount of minutes and back. Though you cannot actually save the time in terms of minutes, only as hour values through the UI.
Ideally we would allow time to be entered directly as hours and minutes and/or allow partial minutes, but right now that isn't allowed by the time entry system.
I would definitely advise that you submit this as an enhancement request though to allow more flexibility with the values that can be entered for time entries. If we get more requests from unique clients then there is a higher likelihood that it will receive attention in a future updated version. Enhancements can be entered from the Submit a Product Idea button on the solutions.teamdynamix.com page.