When the widget is set to show excerpts and display a certain length (in words), I’m seeing two different behaviors.
If the post doesn’t have an excerpt already, the widget generates an excerpt at the predetermined length.
If the post has an excerpt, the widget shows the entire excerpt without trimming the content of that excerpt at all.
The fix is probably a function to apply the excerpt length to all excerpts. Adapting something along the lines from Automatically Shorten the Manual Excerpt might do the trick.