It's cut that way by the builder to lighten the slide weight. My other one has 4 big holes drilled in each side of the front of the slide and the back of the slide has been bored out to take un-needed weight out too. It's about reciprocating mass.
If you get the opportunity to shoot a custom 1911 or 2011 some time, jump at the opportunity. The difference in the feel and reliability can pretty shocking.