ok... I guess the best way to explain it is this. You have two units walking towards each other. One is the Archers. The other is Knights.

You purposely alt-clicked the Archers so you can melee the Knights. However, the Archers still have their bows out the entire time they're walking. It's not until both the Knights and the Archers are at charging distance that the Archers think "Hey, we were ordered to melee and we're still carrying bows. Let's pull out our melee weapons!"

By the time the Archers DO get it out, the Knights have either just smashed into their line with their charge (not hindered by switching weapons) or the Archer counter charge is so disorganized that it's probably worse than not charging at all.

On top of all this, if the Archers happen to be the general for this battle, the actual General himself will not hesitate to charge on time... running FAR away from his support troops who are trying to figure out which weapon to use.