"Units were formed as a 100 recruits and remained that unit through its life. As men died or left wounded the unit shrank."
Ah, no. Not nearly as simple as that. After a battle where senior NCOs had died along with other soldiers, it would be necessary to re-form legionary structures. Replacing trained legionaries with new troops would never be easy and new units would need stiffening with experienced soldiers, so the real strucrture of any legion at any time would be fairly mixed.
One cause of unit deflation is that generals like to have lots of units, and if there are not enough soldiers (or materiel) the components may shrink. This was one of Hitler's problems; as the German army was more than decimated during WW2, formations got smaller and weaker but the Fuhrer didn't get to know. Towards the end in Berlin he was still issuing orders to units that had more or less ceased to exist.