It's just a meter of ES interpretation of the two relationships:

• within matches because Elasticsearch allows an object to be within itself.
• contains does NOT match because Elasticsearch treats "contains" as a strict enclosure, requiring the query shape to be strictly inside, not just identical.

I.e.

• Object A is within object B if it is completely enclosed by object B
• Object A contains object B if it encloses it and there is still little extra space in Object A