According to the RPG weapons rules, a standard infantry rifle round (from a 31st Century high-tech weapon) does a point of damage to 'Mech armor on a roll of 11+, if I recall. That means, you've got to score an average of 12 hits for each point of damage (each hit having a 1:12 chance of doing damage). With automatic weapons being fired by an entire platoon of grunts, that very roughly adds up to the several points of damage that a rifle platoon inflicts.
The game apparently simplifies it drastically, by assuming that each and every trooper either lands 6 rounds on target to do 1/2 point of damage (0.52 damage, to be more exact), if the platoon as a whole "hits", or else every single trooper fails to land enough rounds to do damage, if the platoon "misses".
Personally, I'd rather see reduced infantry damage "on average", but with a table that does variable damage based on the margin of success or failure (unlike the missile hits tables, which use a second die roll). The greater the success, the higher the total damage (up to a maximum of around the current numbers), but with so many shots being fired from so many rifles, a few hits and some trivial damage should be fairly likely even with relatively poor shots.
If that 'Mech steps into short range of your Regulars without a movement modifier (4 to-hit), and your platoon rolls a perfect "12" (margin of success = 8 )*, then the rated 14 damage for the squad is believable (28 guys each hit with an average of 6 shots = 168 high tech armor piercing rounds hitting the target in several tight groups). If the modifiers are higher, and the roll not so great, but the roll still "succeeds" by a couple of points, 1-4 damage sounds more likely, as the smaller number of hits are likely to be spread out across the target. Considering that doing ANY damage with infantry weapons is already a "critical hit", the odds of doing through-armor criticals could be penalized as well.
* = If I don't leave a space between the "8" and the ")", the editor interprets it as an emoticon.