AvgFieldLength seem wrong

I am running an index with bm25 similarity over a field named title.

When I (manually) calculate the teh average field length I find ~10.2
However when I use elastic search explain I get much smaller. This ruins
the score expected calculation

Any clue what goes wrong ?

Example of the result:

{'description': 'weight(title:5 in 72454) [PerFieldSimilarity], result of:',
'details': [{'description': 'score(doc=72454,freq=1.0 =
termFreq=1.0\n), product of:',
'details': [{'description': 'idf(docFreq=3040, maxDocs=901722)',
'value': 5.6922855},
{'description': 'tfNorm, computed from:',
'details': [{'description': 'termFreq=1.0', 'value': 1.0},
{'description': 'parameter k1', 'value': 2.0},
{'description': 'parameter b', 'value': 0.75},
{'description': 'avgFieldLength', 'value': 2.2184565},
{'description': 'fieldLength', 'value': 5.2244897}],
'value': 0.5961232}],
'value': 3.3933036}],
'value': 3.3933036}],
'value': 11.2237425},

*The setting I use : *

{'settings': {'index': {'creation_date': '1430658049103',
'index': {'store': {'type': 'default'}},
'number_of_replicas': 0,
'number_of_shards': '8',
'my_bm25_t': {'b': '0.75',
'discount_overlaps': 'false',
'k1': '2.0',
'type': 'BM25'}}
{'mappings': {product': {'_ttl': {'enabled': True},
'properties': {
'title': {'analyzer': 'english',
'similarity': 'my_bm25_t',
'type': 'string'}
'brand': {'index': 'not_analyzed', 'type': 'string'},
'condition': {'index': 'not_analyzed', 'type': 'string'},

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