On-disk doc_values question

I am using ES 2.3 version and below fields are not-analyzed.

Operator: {
type: "string",
index: "not_analyzed"
},
Route: {
type: "string",
index: "not_analyzed"
},

why fielddata api is showing these fields are using heap.

_cat/fielddata?fields=*&v
id host ip node total log_component Operator Tail Route Fleet
QLsdcLxGS-etcJs4hEzi2A 127.0.0.1 127.0.0.1 Nightwatch 124.9kb 3.8kb 11kb 39kb 60kb 10.8kb

and I have another field which is analyzed and but it's not shown in fielddata api above.

log_detail: {
type: "string",
analyzer: "snowball"
}

my assumption is on-disk doc_values fielddata is not stored in memory.