it is possible that the school is thinking this way:
Skill subjects like math and language arts are sequenced with one level building on the last one. Teaching in order is important.
Content subjects like Bible, history, and science are less sequenced. These can be taught out of order.
Looking at the Scope and sequence for 5th / 6th grades:
History:
World History in 5th Grade
New World History in 6th grade.
Science 5th: insects & plants, Mammals, energy:light, Geology, Oceanology, Energy and Engines, Astronomy, Weather, Vertebrates
Science 6th: Plants, Invertebrates, Geology, University / Space, Matter and chemistry.
Bible is probably similar.