I always thought it was because of the peculiarities of approach flap generated wake in the 757 that means, according to the CAA, that it is treated as a heavy when following on approach, but not for departure.
Now what you do as a matter of airmanship is entirely up to you.
One point though, if you are about to follow a 757 onto the rwy and require 2 mins (for whatever reason).............. tell ATC nice and early.