I am very old, but I only learned this year that hotel buffet scrambled eggs are usually made from powdered egg. That explains why they are a uniform colour. Scrambled eggs made from scratch have yellow and white bits.
On the plus side, I also learned recently that the chicken (or at least the "Original Recipe" pieces) at KFC is made from scratch in the shop every day from fresh chicken pieces, flour, and herbs'n'spices. I had imagined it was all done in a factory somewhere and then cooked from frozen.
What are some other bits of restaurant knowledge that surprised you?