Borders do matter. Dark borders cause deficiencies in black level to be more obvious, while white borders accentuate deficiencies in brightness. In the below image, the gray/black square in the middle of each border is the exact same color.
The same goes vice versa. But when the display is off you have true blacks but the white of most internet pages won't ever match the white of the bezel.
And it's an optical illusion that only works well with a small area surrounded by a bigger margin. Which is not true for the iPad.