I don't pay much attention to what units I get, sorry. Between the weekly troop influx from finishing GE, a pair of Governor's Villas from the last two summer events, emissaries and Royal Bathhouse, I get enough so that it is pretty much evenly spaced between types. I haven't used any barracks since sometime in the middle ages, and I'm in Space Age Mars at present.
However, I sort of noticed something to this effect on a different server (my primary world is Sinerania, but I have five others). On one of my other worlds I am in Arctic Future like you are and I have somehow stumbled into a spot where I have three recurring quests, so I leave one of the recruit recurring tasks up . . . I figure maybe I'll get the diamonds from it before I age up if I cycle it once a week or whatever. I routinely am waiting for 1-2 days to finish gathering the non-heavy portions of the quest, whereas the heavy troops take upwards of a week (similar set up to my Sinerania world but without the Royal Bathhouse . . . i.e. still no barracks). It should be random, but I also wondered about the general dearth of heavy troops when I am expecting an even spread over time.