Displays the excerpt of the current post after applying several
filters to it including auto-p formatting which turns double
line-breaks into HTML paragraphs. It usesget_the_excerpt()to first
generate a trimmed-down version of the full post content should there
not be an explicit excerpt for the post.
get_the_excerpt() applies the get_the_excerpt filter, to which wp_trim_excerpt() is attached.
If you look at that function you’ll notice that excerpt_length is only applied when the excerpt is generated from the post body automatically. Otherwise, the manually written excerpt just passes straight through.
Honestly, since you are writing the excerpts I don’t understand why you want to automatically truncate them, but if you must you can add a filter to wp_trim_excerpt using wp_trim_excerpt() as a model.