It depends on the philosophy of the school. Theoretical CS is often considered a branch of mathematics, e.g. graph theory, number theory, etc and thus a school will make it part of the school of science. But other schools that have an active engineering department will go ahead and make it part of the School of Engineering. IMO, the undergraduate curriculum is probably more engineering-oriented than theoretical while graduate CS is definitely more theoretical. It is effectively a distinction without a difference because whether CS is designated as part of the engineering program or part of the science program you are still going to stay up till 4am debugging.