I had a similar problem years ago with a Gracy Trimmer. If your resizer is not consistant your timmed cases will not be consistant. I see variations in my resized brass caused by hardness , amount of lube , neck thickness and a host of other problems but mostly neck tension when pulling the expander ball through the neck. If you have a Micrometer case gauge you can see the variations. If the length from the case base to the shoulder varies then will the case length vary using a trimmer that measures from the shoulder.
Strive for better consistancy when resizing your brass and then trim within those variations.