It is just how the cooling jackets are designed. Some say closed is better and others say open is better. I run an open deck on an all motor build pushing 11.7:1 compression and haven't had a single issue yet.

A lot of people will say that open deck is an inherent weakness due to the thickness of the cylinder walls but I have yet to hear of anyone who has had a failure due to this. I suppose if you abue the piss out of your car it could be a flaw, if you take care of your junk it should be a moot point.