It's pretty long; but hBbomberGuy has an interesting video on how(at least so far as could be inferred from externally available interviews and GDC talks) Human Revolution came to have its somewhat peculiar attitude toward ammo.
It's an interesting watch; but if you don't have 3 and a half hours the TL;DR is that the team identified combat early on as something they wanted to improve, and did; but discovered in playtesting that making just shooting people smooth and robust tended to crowd out the other elements(stealth, map exploration, vent-sneaking, etc.) that they also wanted the player to pay attention to.
They ended up exhaustively mapping out all possible ammo supplies across each mission in order to attempt to restore balance by imposing fairly tight resource management limitations on the otherwise basically-no-reason-not-to-use-them guns.
Speculation is that they(probably correctly) saw the original's frankly pretty clunky and deliberately player-character-stats-dependent combat as a no-go for console and general audiences; it is not known why they tried to get combat back under control via ammo rather than via health(which just regenerates as long as you don't get outright killed) or via the original's location based damage and capability degradation(get shot in leg, lose move speed, etc.) which helped make combat something that you had to have a real think about getting involved in; even if you specced out an agent who was actually competent at it, which was optional in the original; and if not done would leave you with a character who was a real liability; unlike Jenson who is a solid marksman by default.
