BF2 to Acr furnace auto-upgrades with upgrade tool via unity

Using the upgrade tool, if you click to upgrade a BF2 to an arc furnace, it auto completes spending unity without actually using the unity apend option (ie the unity is spend as soon as you click with the upgrade tool icon)

Isn't that how it's supposed to work?

Arc furance I is actually cheaper than BF2 (180 vs 160 cons.part III i recall)
so brobably its because already enough materals are conciderd "delivered" to the site.