I can understand this for 1/2 sizes but in some cases we are talking 2 or 3 sizes out. Given a size is around 1cm, I've yet to come across anyone who has a toe 3cm's longer than any others! My personal experience with this (fitting skates for many years) is a misguided belief around comfort, to quote - "I chose that size because the skates didn't hurt when I laced them up". This is because they are so long and wide there are no pressure points on those little foot bits that don't quite fit within a retail skate profile.
As to comparing shoe size to skate size, one of my great bug bears in life. Everyone wears shoes differently and shoe manufacturers still don't follow consistent sizings, I just don't get why this keeps getting repeated verbatim instead of - your feet measure 26cms long so you start with a size 6.