Spell gem level seems to not scale anything

Finishing act 2, I acquired the Doedre's Tenure unique gloves. After equipping them, I found very little difference in damage. I tried to investigate: '100% increased spell damage' modifier seemed to work correctly, but I still had 167 dps on a level 9 frost nova skill. Then I decided to try to decrease level of frost nova by unspecing 'Endless blizzard' node on the skill tree. Numbers on a tooltip didn't change. TTK did not change. But when you hover over skill gem, it says that base values did change.