sorry if this is a dumb question, but how is this possible? what can cause the cache hit rate to change just by changing the model?

This is based on what we measure from the API - we see different endpoints depending on how they are served can have different cache hit rates (cache hit rate is defined as the % of cacheable tokens that actually come back as cache hits)

One possibility (just a guess): new arch produces a more compact cache, it becomes cost effective to hold more cache for longer.