Is MS Project changing my Work Hours?

Possibly.

This is most commonly due to rounding differences. For example, MS Project defaults to Work adjustments as a percentage of a workday, instead of work hours. MS Project won't always allow a partial percent of a workday in a calculation. A simple example is that 1 hour actually equals 12.5% of a normal 8 hour workday. Doing a simple 1 hour work assignment to a resource may round the work up to 13% of the resources workday. This rounding on the percentage then allocates 1.04 hours instead of simply one hour when calculated without rounding.

Project Insight and Microsoft Project are not identical products with identical mathematical calculations. MS Project often rounds at the task level prior to calculating the sum of the entire schedule while Project Insight calculates over 10 decimal places prior to showing rounded, user-friendly numbers as totals in the interface. Project Insight was designed to provide real-time resource allocation data across all projects and resources. Yay for better accuracy!

While the above example may seem minor, these rounding differences can add up to larger differences when working with multiple tasks. Exporting and importing task data over and over can mean that work and schedules are calculated from one program to the next over and over.

You can avoid some of these problems by entering duration and work as whole numbers. The problem is most frequently seen when work is calculated based on duration and resource percent allocation. Simple, whole number values used in resource allocation percentages and duration often don’t require crazy work value calculations.