Flour conversion chart

When baking and using substitutes for wheat flour in recipes you cannot substitute certain flours with wheat flour on a 1:1 basis - or so I'm told. To be honest, I'm unsure why as I've never been brave enough to try and convert a recipe to using a gluten free alternative. 

For those of you brave enough, here is a handy conversion chart. 

Personally, I prefer the White Wings gluten free flour as it can be used on a 1:1 ratio as a substitute for wheat flour. 

If anyone can enlighten me on why you can't use other flours on a 1:1 ratio I'd gladly appreciate it!