This is almost certainly because there is a mismatch between the number of hours you have declared to be "in a day" (probably left as the default 8) and the number of hours you are actually working in a day (probably defined in the Working Time as 7 or 7.5).
Firstly set your working hours properly. Then set the "duration" of a day to equalt the number of hours worked in a day according to what you have set in the Working Time.
Unfortunately you will have to go through all tasks and reset their durations as fiddling with the above will have recalculated the durations of all tasks.
This is a common mistake with MSP and the way around it is to ALWAYS set up your working calendars and working times when you set up a new project and definitely BEFORE adding any tasks...