Dota – Why can’t Kunkka equip a Quelling Blade

This might have something to do with the static differences with the map layout. They may be small differences but definitely adds up as the match progresses on between the two creeps.

From this article.

Mid lane in Particular

Mid lane, as opposed to top and bot, has a slight difference in creep arrival, as a result of pre-discussed enlargement of scourge base. This means that the sentinel ranged creep is standing up-hill, and a ranged hero will be able to actively use the Fog of War, thus having a huge advantage in the first minute of the lane as the enemy do not have the ability to predict when the attacks are landing.

Ghouls and Treants

Although these two units are alike in almost every case, there is one small difference; Treants have a greater base than the ghouls, due to the greater unit size they have. This in combination with the Treants coming from the lower left corner results in Ghouls being able to 'hide' behind the Treants, making them a lot harder to hit.

The positioning of the towers and also the slightly larger base of the scourge base might also affect the way the scourge's creeps arrival as they spawn on their base.

They do not stack.

There's also seem to be a bug in the sense if you have 2 Quelling Blade in your inventory and drop one the game removes the bonus as if you didn't have any.

More info here