As I understand it the brain does a lot of 'rewiring' and reorganising in the first few years. The exact way it organises things would depend on genetic factors and on environment. Once it has done so you would find a natural aptitude for math at the expense of say artistic ability. Good cordination, but poor memory. If you are lucky the things you are potentially good at will outweigh those things you will struggle with.
You could still learn most things with intensive training, but in your 'poor subjects' it will always be uphill.
If we all just did what we are naturally good at that would be ok, but there are skills we must have to get by such as literacy, numeracy and the ability to organise and plan. If those are our weakest points we need all the help we can get to reach a basic standard.
That means that there are many kids who need extra help even though they do not have a specifically named disorder.
It 'could' also mean that specifically named disorders such as dyslexia are in fact extreme cases of the above rather than seperate problems.
Not that it changes anything if that's the case. It still comes down to identifying and helping those who need it most, but perhaps not drawing so many lines between kids with varying abilities.
Whenever I see a line I think of those who were just a little way the other side of it.