Why does final pH value of buffer solution after addition of acid not depend on amount added?

I'm brushing up and the example in my book is against my intuition.



> 
> A buffer solution contains 1.0 mol/L acetic acid and 1.0 mol/L
>  sodium acetate. The solution is at pH = pKa = 4.75. What's the pH
>  after addition of 0.01 mol/L HCl?
> 
> 
> 


They don't write how much of the acid is added.


They go on stating that addition of x = 0.01 mol/L H+ results in pH = 4.75 - log[(1.01 mol/L) / (0.99 mol/L)] = 4.733.


That's all fine from the formalism. What's against my intuition is:


Why is it not required to state **how much** of the acid was added? Given the buffer solution, is the outcome of adding acid the same independent if I add a Liter of if I add a mL?


