Another factor comprehension does not share with the core VCI subtests on the WISC-V is the amount of language comprehension necessary to answer the questions. Similarities and Vocabulary both consist of very brief stimuli (one or two words), which may make a difference for some children.
Also, Co is somewhat related to personal experience, unlike the other three verbal subtests, which are more academic in nature. It may be that a six-year-old hasn't had sufficient experience of the world to have encountered some of the situations addressed in Co.
