CachingAllocator Vs CachingHostAllocator

Hi, I am currently reading the code of cachingallocator and cachinghostallocator, and I found the difference between them is that the cachingallocator owns a chain of Blocks and it will merge the blocks when allocator frees. Why not the cachinghostallocator follow the same pattern as cachingallocator does? It looks like using more cached pinned-memory.