First I want to mention that it's possible for a mod or just in the game to have a lord use mostly from the best units, to make for example a mostly knight/mounted army. The original way (I miss Zendar) was to have a lot of low soldiers, fewer of their promotion and fewer of the next etc. I also wonder if you might misunderstand the 'tier'. A lord usually has only 1, max 2 tier 1 units: recruits, any promotion is a higher tier. If there are multiple prootion path, distribution of forces is equally spread through both. As many swadian sargeants as knights, for example. I think it is SoD special (not vanilla) that some promotions move directly to units that are much better, sort of skipping a tier.
This all is my guesstimate, no hard numbers or science, understanding of the game or anything. It doesn't take into account any special programming for specific lords (currently playing a mod that has 3 types of lords: infantry, cavalry or mixed, cavalry lords have about half the amount of units as mixed, infantry lords have a bit more) or 'bodyguard' units.
It's been a while sine I played around with the difficulty settings, but iirc there wasn't a hard or easy setting for it. The difficulty was in units actually fighting better, aiming being harder etc. I remember laughing at peasants with spears, untill I raised the AI and they started timing things better and consistently step aside from my lance charge and proceed to stab my horse out from under me. It's a constant dilemma. Get a great lance for the huge reach so my horse doesn't get stabbed in the face, or get a glaive or similar so I have a good weapon in hand already when my horse inevitably gets stabbed to death. Or keep my distance and shoot arrows.