My interpretation would be that 'everyone' has the man rune, so 'everyone' has 4 runes - man, and three others, but we only mention the three that are varied. They are humanoid after all, (assuming they are). This gives the possibility of "losing their humanity" becoming beast or chaos monsters or whatever. It also ties in with previous Glorantha lore (some trolls losing their man rune and becoming non trolls frex) and would allow an easy definition for magic affectiong humanoids (ie thoe with the man rune). Personally, if I were building this thing up from the ground, I might have trolls having darkness and man automatically, elves having plant and man, mostali having stasis and man, etc etc. OTOH, I'd be easily persuaded that everything is made of everything and that everyone was a mix of runes 5%man, 1% earth, 6% illusion etc adding up to 100%. but can see that three being picked out for easy gaming notation, rather than having a rating in each one is easier and more fun.
Keith