Sum with negative fields (double type) strange result

There is someone that could me explain why by this simple aggregation:

    > GET cfo1/_search
    > {
    >   "size": 0,
    >   "aggs": {
    >     "NAME": {
    >       "filter": {...},
    >       "aggs": {
    >         "somma": {
    >           "sum": {
    >             "field": "SommaDiOre_mens_Comm_CdC"
    >           }
    >         },
    >         "aaa": {
    >           "terms": {
    >             "field": "SommaDiOre_mens_Comm_CdC",
    >             "size": 12
    >           }
    >         }
    >       }
    >     }
    >   }
    > }

I get this response with the sum that is not zero:

    >   "aggregations" : {
    >     "NAME" : {
    >       "doc_count" : 11,
    >       "aaa" : {
    >         "doc_count_error_upper_bound" : 0,
    >         "sum_other_doc_count" : 0,
    >         "buckets" : [
    >           {
    >             "key" : -141.96,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : -41.07,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : -13.6,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : -6.83,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : -6.0,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : 1.44,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : 5.39,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : 6.0,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : 13.6,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : 41.07,
    >             "doc_count" : 1
    >           },
    >           {
    >             "key" : 141.96,
    >             "doc_count" : 1
    >           }
    >         ]
    >       },
    >       "somma" : {
    >         "value" : 1.5987211554602254E-14