In the current system, ilevel is a fixed property of an item, and it determines the maximum to which you can adjust its stats. What you're asking for is for the game to calculate the ilevel according to the item's current stats. That's a very different system, and I doubt it would be easy to change it.

That said I do agree the Recommended button sometimes has some strange ideas on what gear is best. Like it keeps wanting to put the aetheryte earrings on my level 1 characters when the Ala Mhigan earrings have far better stats (as the aetheryte earring syncs to level 1 stats while the Ala Mhigan earrings are fixed at +5 everything).