When in a multiplayer environment, if another player sneaks, and you also sneak, their player model is visibly lower than yours by about two skin pixels. However, when standing, the model heights are the same. Basically, a player model always appears to be the correct height to the user, but other players see them as shorter, and vice versa. Refer to the attached images for a demonstration.
Have two accounts in a multiplayer server and shift inside each other. Notice that on each screen, the other player appears lower.