That is probably due to the wind. FSX is not that good at interpolating the wind. You can set "Smooth Wind" via FSUIPC or activate "Direct Wind Control" in FSrealWX. There is also a difference in the "Weather Mode", "Direct" or "Weather File".
If you find an error, then please give me exactly about your system. Which sim (FSX / P3D / Xplane). Which connection (FSUIPC / SimConnect / FSrealWX-Bridge). Which OS.