Comment by LorenPechtel

They don't know, but we can be pretty confident that it's no more than a trace.

If eating the wrong thing can kill it makes no difference, "may contain" is the same as "contains". However, if eating the wrong thing will simply mess up your day "may contain" is a very different thing than "contains".

And if I learn I react to product X so be it, I simply don't eat X.