Lucene Total usage < Fixed Bitsets usage?

When looking at the index memory statistics in Kibana, I currently see this:

My question is pretty simple: How come the "Fixed Bitsets" memory usage is bigger than the "Lucene Total" memory usage, even though the former is supposed to be counted in the latter (according to the explanations in the popup).

Retrieving the index segment statistics obviously reveals the same thing, i.e. fixed_bit_set_memory_in_bytes > memory_in_bytes. I have troubles explaining how this is possible. Is the legend in the popup simply wrong or is there a metric computation bug somewhere?

{
  "_all" : {
    "primaries" : {
      "segments" : {
        "memory_in_bytes" : 14774894,
        "fixed_bit_set_memory_in_bytes" : 21090152
      }
    },
    "total" : {
      "segments" : {
        "memory_in_bytes" : 28448222,
        "fixed_bit_set_memory_in_bytes" : 41924824
      }
    }
  },
  "indices" : {
    "my-index" : {
      "primaries" : {
        "segments" : {
          "memory_in_bytes" : 14774894,
          "fixed_bit_set_memory_in_bytes" : 21090152
        }
      },
      "total" : {
        "segments" : {
          "memory_in_bytes" : 28448222,
          "fixed_bit_set_memory_in_bytes" : 41924824
        }
      }
    }
  }
}

Thanks in advance for your help!