Working with the pde numeric solvers.

If I use a statement like:

sol = NDSolveValue[{op == 0, pbc, Subscript[\[CapitalGamma], Dc]}, 
  u, {x, y} \[Element] \[CapitalOmega]]

I will get back an interpolating function and I can do things like sol[3,4] to pull values from the function.

If I use the form

sol = NDSolveValue[{op == 0, pbc, Subscript[\[CapitalGamma], Dc]}, 
  u[x, y], {x, y} \[Element] \[CapitalOmega]]

I get back an interpolating function with [x,y] appended at the end. If I try sol[3,3] the output just appends [3,3] to the end.

How ever statements like: ContourPlot[sol, {x,-1,4},{y,0,4}] still work.

Can someone point me in the right place to understand why the two outputs are different?

Thanks