There are two things that don't add up, and I'm honestly not sure which one is the bug.

When looking at your DPS in the inventory screen, Magic Weapon's damage increase also affects weapon speed, in addition to the straight 10%-15% damage increase. The increase is not shown when looking at weapon speed in your detailed stats however.

I figure either Magic Weapon shouldn't be affecting weapon speed, in which case fix the DPS calculation. OR, Magic Weapon should display the weapon speed bonus (and people would probably like it if that was mentioned in the skill calculator or power description too)