Question: Index in 1D math input


if I write N[0]:=1; and press enter, I will get the response

N[0] := 1

Now if in my large document I happen to have defined a variable b:=5; then later 
I try to define another variable N[b]:=1; and press enter, I get this response:

N[5] := 2

But N[b] has nothing to do with "b". Why does Maple do this? Is there 
some concept I am missing? In what context is this useful?

many thanks.
Please Wait...