The Golden Gun is always a one shot kill, armored opponent or not.
Headshots aren't broken in any way, only a few weapons kills in one headshot if unarmored: the Cougar Magnum, Silver and Golden PP7, Moonraker Laser, shotguns if close range. These will kill in two headshots if the opponent is armored. Same applies to other guns, Klobb = three headshots to kill if unarmored and six if armored. (This might vary with other guns but it really depends on the health of the opponent and other circumstances, but this will help you count the required headshots to kill)
You're probably not taking into account the invulnerability time, wich means after you hit someone they'll have a short period of invulnerability time (Like in the original game) where they can't take any damage, only the shots that get the hit confrimation sound are the ones that dealt damage. The less health they have, the longer the invulnerability time is. Same for you.