Relevance impacted by range clause


When I try this query

    "bool": {
        "must": [
            {"range": {
                "lt": "2000-01-01"
            {"terms": {
                "directors": ["Ridley Scott","Billy Wilder"]

My score is impacted by the date range query. Ok I understand that I can make a filter to avoid this. But my question is how the relevance is computed for the range query. If I try to make a query with only one range clause like below, all my documents have the same _score (1).

        "range" : {
            "date" : {
                "lte" : "2001-01-01"

Can someone explain me how and why the score is impacted by my range clause in my first query?