In landing config, the flight control system tries to maintain the trimmed angle of attack. When you trim, you are setting the angle of attack for the computer to hold. Thus increasing throttle increases speed, which decreases angle of attack, So the computer compensates by pitching up slightly to maintain the angle of attack you set by trimming. Thus it pitches up and increases altitude. Same when you decrease throttle, it increases angle of attack causing the computer to pitch down to compensate, causing you to reduce altitude.