Why RocksDB metrics cache-usage is larger than cache-capacity

[Lei Wang]

I enable RocksDB native metrics and do some performance tuning.

state.backend.rocksdb.block.cache-size is set to 128m，4 slots for each
TaskManager.

The observed result for one specific parallel slot:
state.backend.rocksdb.metrics.block-cache-capacity  is about 14.5M
  state.backend.rocksdb.metrics.block-cache-usage      is about  24M

I am a little confused, why usage is larger than capacity?

Thanks,
Lei