It's all very simple.
Long before there was a JAA/EASA/FAR....there was CAR4B, which established 400 agl as the end of the second segment.
Carryover from the old days...and a good one at that.
However, with piston transport aircraft, 50 feet was the screen height, yet 35 feet was established as the screen height for jet transport aircraft, by CAR4B
Why, you ask?
That question is a bit more complicated, however...the basic facts are that the performance of the early jet transports was many times so poor, and the runways available so short....that it was impossible for the 50 foot screen height to be achieved, at the max weights anticipated.
I flew one of these early poor performers for quite awhile, the B707-320, and at its max weight (317,000 pounds), up close and personal with the far end of the runway whilst becoming airbourne, was a daily occurance...with all 4 turning, let alone with one engine unserviceable.