I see what you mean, but that is how Adendum is designed: to get that extra level. I won't argue SE squinched in the last of lvl IV nukes right in with BLM at 75. Now, though we are on track for Thunder V at lvl 99. RDM have Tier IV nukes 100% of the time and they have built in MAB which we have to sub for. If level balance is your only issue, then make the descriptor of the spell so that SCH needs Addendum until it learns the Tier V version. It doesn't, however, make sense to have to go from Tier III to Tier V requiring the Addendum stratagem to cast.
It's obvious SE is trying to readjust a game which has been lvl 75 centric for most of its life span. They are adjusting things little by little, step by step. I just want this to be laid out as something I see. It doesn't have to be changed until the framework is in place, but this is one of those odd instances where power creep hasn't been readjusted post 75.
Tangent: Level balance? I haven't been in a traditional leveling party since... Welcome to Abyssea, where 30 is the new 95! (I WILL get flamed for this... Make Aby lvl 75 requirement, then level balance will matter between levels 30-75+.)