Search scores are generated for full text search queries, based on the statistical properties of matching terms, and ordered high to low in the result set. Query types that aren't full text search (wildcard, prefix, regex) aren't ranked by a relevance score. This behavior is by design. A constant score allow matches found through query expansion to be included in the results, without affecting the ranking.
For example, suppose an input of "tour*" in a wildcard search produces matches on "tours", "tourettes", and "tourmaline". Given the nature of these results, there's no way to reasonably infer which terms are more valuable than others. For this reason, term frequencies are ignored when scoring results in queries of types wildcard, prefix, and regex. Search results based on a partial input are given a constant score to avoid bias towards potentially unexpected matches.