Also, there was no GOOD reason to only load 18 rounds in the mag, but here's why it happened:
In the beginning, M193 ammo was issued in boxes, loose. GIs would typically dump out the boxes into their ponchos and proceed to load their mags. Sometimes, they would overload the mags, as you can force 21 (and sometimes 22) rounds into the mag. This is BAD, of course, and can (and usually will) cause a failure to feed. This was eventually fixed by issuing the ammo on loaded stripper clips. 1 mag = 2 clips of ammo.
The other problem was that GIs would disassemble their mags for cleaning and remove the follower from the spring. Because the spring connected to the follower in almost the center, it was easy to put the follower back on the spring backwards. You wouldn't notice a problem until you tried to load the mag. That's when you'd notice that it was difficult to put more than 18 rounds in the mag, and if you forced the last two in, it would bind the follower and you'd have no spring pressure to feed the ammo. This was "solved" by instructing GIs never to remove the followers from the spring (though of course I'm sure it still happened sometimes).
By '68 or so, most units were correctly filling their 20 round mags with 20 rounds, and not having any problems. But some units, who were trained the "old" way and/or who believed all of the myths and mistruths that were spread about the M16, would still train the "18 in a 20" method. Some folks STILL underload their mags to this day, and there is NO reason to do so.
Well, okay, there is ONE reason that makes sense:
Underloading your mags by ONE (1) round will make it easier to to a "tactical reload", which is where you remove a half-empty mag and replace it with a full mag during a lull in the fighting. When you do this, your bolt will be forward on a loaded chamber, and if your mags are full, you have to give the mag a FIRM smack on the bottom in order to compress the spring enough to allow the mag to lock into place. If you fail to do this well enough, the mag will often fall out of the gun on the next shot. Underloading the mag will leave more slack in the spring, and make reloading with the bolt forward a bit easier.
I still prefer to load to capacity. As long as you understand the big picture, this is no problem.