When flying S&L, thrust and drag cancel each other out, as do weight and lift, hence no acceleration.
Now, if I point the nose up, a part of the thrust vector points upwards, and (once all the other forces have settled down and we're in a steady climb) there's again no acceleration because I'm in a constant climb. Therefore since the plane hasn't got any heavier, there must be less lift generated from the wings than in S&L since the total upward force is lift + upward portion of thrust.
Now, I point the nose down and, (if I'm not idling the engine totally) after everything settles down, I'm again in a zero acceleration situation. Since part of the thrust vector is down (assuming I am actually pointing the nose below the horizon) then there must therefore be more lift generated from the wings than in S&L.
I understand it, I can explain it (well, you tell me...) but it still seems wrong!