Well I'm sure Festool isn't doing 100% inspection of all the rails they produce, they're probably using typical AQL charts which basically say "at the AQL risk level of your choice, and with "X" amount of received parts, you need to randomly select and sample "X" number of those parts. And of that number sampled, only "X" number can fail.
So inherent in the inspection process, it is known that some outliers will make it through inspection. Short of 100% inspection, it's just a risk everyone takes. Everyone tries to minimize the risks, that's the reason it's imperative to choose the correct AQL level. If there are problems in the field, a Quality Assurance group can/will change the AQL level or they may demand 100% inspection of critical dimensions...but 100% is soooooo expensive.
Besides, everything changes, the tools wear, the tooling wears, the composition of the material itself may change slightly, the process can have a hiccup...it all adds up in the end.