There are multiple theories for this. The popular one is that we are simply not modelling initial conditions correctly. However, our experimentalist friends argue that the Schmidt number, which measures molecular diffusivity, might have significantly more importance than people assume. This would complicate things, because our best simulations are *orders of magnitude away* from modelling real Schmidt numbers accurately!
The system I am referring to is the "Rayleigh-Taylor Instability" (RTI). Pour creamer into coffee, and watch the plumes descend - a heavy fluid falling into a lighter one - this is an RTI. Simulation predicts that this process should be much *slower* than it actually is.
Why??