Move the mouse over a deactivated tab. Notice how it fades in.
Move the mouse away from the deactivated tab. Notice how it fades out.
Now, move the mouse over the active tab. Notice how it plays the same animation as a deactivated tab (starting from a darker color than an active tab should ever be). The animation looks wrong, as if it is flickering.
The same problem occurs when you move out of the tab (it fades into the color of a deactivated tab, then "jumps" to the correct color).