You can select multiple AI strategies for a unit type, but you are saying that just qualifies the unit type to be made by the AI for the purpose of any individual unit forever playing that role, rather than that each individual unit can be used for each of its different roles at different times. One Legionary (both strategies checked in the unmodified game) will not start as an attacker, take a city, then sit and defend it. Either this was an attack legion and he has to go find another city to attack, leaving the defense to perhaps a legion that was born to defend, or else this is was a defense legion, in which case it would never have attacked a city to begin with.
How could you test this systematically, as opposed to anecdotally?