There is often a big difference between the sample rates of parameters on the QAR compared to the FDR. The actual sample rates used on the QAR can also vary between different aircraft types. FDM software often interpolates the missing data to match it with the data of a higher sample rate. If the air/ground switch is sampled at a lower rate than the normal acceleration this type of interpolation will affect your results. It is always important to consider these issues when analysis QAR data: what does the FDM software do with the data, how does the raw data looks like etc.