There are a lot of different methods and tools that pilots use to determine density altitude. We can't know how your calculation is made, but many calculators don't take into account barometric pressure or humidity. ForeFlight takes a comprehensive approach to calculating density altitude, including factors such as temperature, barometric pressure, and humidity. The result is a very accurate density altitude calculation. If there is a small variation between what we show and what you calculated, it could be related to the rounding of values during the calculation process.