I do not know why the majority of us attach the hairpins wrong, I have done it, and most of us have done it (because it is not just me who is also blonde on the inside?)
There is something in our brain that wants to make it more logical for the wavy side to be up, but you know, then you have it upside down!
I myself was shocked when I heard this but when I then tested it was actually a real difference! The hairpins fit much better if you have them in the right direction, ie with the wavy side down :)
You who did not know this, try it! You will notice the difference immediately :)