Having the larger ejection port helps prevent detonations when clearing the firearm of a live round. It will not hang up on the way out as easily. If the cartridge hangs it may then slide down the bolt face and the primer can then be struck by the ejector causing a detonation.

Ruger would do this because any 1911 is made to take rounds, but there target model is made for thousands of rounds of target and competition shooting. Due to that fact when shooting competition you want the ejection port a hair larger so that you don't have any jams, because one jam or one shell doesn't come fully out and your over, and out of the competition. So they make the gun like this so that it has a little more clearance for all brands of ammo.

My guess would be cost of production