If the sample data isn’t fully loaded then it is possible some sounds will have a slight delay which could be what you are probably seeing. In the FMOD Settings in Unity you can choose to load all sample data or if you want to load banks in game via script it can be enabled there as well.
If using streams, it is difficult to guarantee that all streams play in sync since it depends on external hardware. We have an extra schedule delay that is applied for streaming events in that case, but if there are many tracks, then there can be lots of data that needs to be streamed in.
Generally, setting the events priority to highest in Studio should stop them drifting apart.