Because... at level 71 the new rank of Inner Fire has +95 Spellpower built in. Improved Inner Fire increases that. At level 80, Inner Fire gives +120 Spellpower, and if you improve it, you get +174 Spellpower.
If only we were mages/warlocks and never had a bullshit system for inner fire (Charges and dispellable; double dipping in armour buff negatives).