So in mainly the older FPS games (particularly those made with Source and GoldSrc, as well as some older idTech engine games) players could gain extra height to their jumps by crouching mid-air. Kinda like one does IRL by jumping and tucking in their legs.
And the same thing worked in your game. Jumping and flattening the character mid-air let me climb higher and get into places I wasn't meant to get to.