From paragraph six, "We are also making bars 1, 2 and 3 a bit taller so they will be easier to see."
It's a psychological thing. A few extra pixels do indeed give the illusion of better network reception, albeit the actual level doesn't change.
Similar to designing progress bars, the actual progress is hidden from the user. Instead, the user is presented with the illusion that the process is progressing faster than it actually is.