I wondered that to start with, especially when I was told that 20 calories = approx 1 syn. But syns are much more than easy ways to keep a tab on calories. You get 15 syns allowance per day to keep under for you to use on treats and "nicer foods". You also have a wide variety of Syn free foods which you can eat without having to count towards you 15 syn allowance. You get a handbook that tells you what is syn free
I'm making sound more complicated than it is. It really is quite easy.
Simple terms: You can't compare the two. Counting calories is about reading labels and feeling bad when you have a treat. Slimming Word Syns are more about educating you to eat right, and the right combination of foods