I think the Pictish Warrior is a spearman replacement, and I don't think they scaled the combat strength as linearly as Monthar said. My speculation is that the HP boost is primarily to increase damage "fidelity" (which would still make them last longer, no more 10% minimum damage) and the later units got a larger strength boost to accommodate the WWI era stuff.
Three times: "THIS!"
Even more, I've got the impression that people mix up the relation of health increase and the increase of strength. I think, there is *no* direct relation between both!
First of all, we don't have any indication that increasing the unit's strength will be linear - even more - it wouldn't be reasonable neither. (The "11-strength-Pict Warrior" even may indicate an other "stretching formula".)
- A unequal stretched strength range will allow WWI units to fit in, just as Scarpa mentioned.
- Secondly, this will allow less wired battle outcomes, if early and modern units fight each other, as damage can be more gradual now. (*This* goes hand in hand with the new 100 HP after all.)
But this is *not related* to the mentioned "longer lasting battles"!
- Damage is not dealt by sheer strength, but by the comparison of the attacker's and defender's strength. So, you even could have a strength 500 unit - as long as it fights against their exact counterpart, it will just deal (recently) 50% of the max health damage.
- To grant longer battles, all you have to do is to reformulate the "damage math".
Recently, two units equal in strength will do 5 HP of damage to each other (so, 50% of their max. health points). To make battles longer, you may just lower the max. damage percentage to 30% or 40% (or whatever you might think as adequate).
- Again, the 100 HP just will grant the possibility for more accurate results. Not more and not less!