Questions about the gameplay of Heroes 6.

I was just trying to level up the Assassin's Blade (to level 5) in HotS. I made it to 18 months. I attacked 130,000 succubi. I destroyed them. I look – the experience only increased from about 5,600,000 to 5,700,000. Something's not right, I think... I attacked 6,000 cyclops. Either no experience was added at all, or just a tiny amount. What's going on, people???
I remember when I was leveling up the Staff of Oblivion, it was much faster. But I was leveling it up in a campaign.