When you enter Hours for time entries, that gets copied to the Client Hours field too, but the value can sometimes be slightly different. The difference is more visible after it accumulates in the running totals of the Hours and Client Hours at the bottom in the Time Entries screen.
This happens if you have checked the option 'Apply to Client Hours only' for 'Smallest time increment for time entry' in Global Settings > Time & Expenses. This option applies the smallest time increment in Core to client hours only and not the actual hours being recorded. For example, if the smallest time increment is 0:15 (15 mins = 0.25 hours) and you enter 7.21 in Hours, it also copies to Client Hours, but on saving the time entry, Hours still shows 7.21, but Client Hours shows 7.25, changing to the nearest value allowed by the time increment.
To make the Hours and Client Hours fields consistent, you should un-check the option that applies the time increment to Client Hours only. Check the Core Help for details.