The Lee Trimmer trims to SAAMI length. Ideally, you would want an adjustable trimmer so you can trim to the "Trim Length" as recommended in reloading manuals, the idea being that you trim the cases back to the recommended, case specific, "Trim Length". This allows multiple firings / reloads before the case extends back to SAAMI length and needs to be trimmed again.
The variation in the case lengths you document is likely due to slight variations as you screw the case specific "Case Length Gauge" into the "Cutter and Lock Stud". You could try gently sanding the pin on the Case Length Gauge so as to reduce it to, or close to, the shorter Trim Length.
As far as the variations in length using the Lee Trimmer, try to screw it in TIGHT to achieve some sort of consistency.
Alternatively, keep your Lee Case Trimmer to test for cases that have stretched to SAAMI length and then trim them with an adjustable trimmer (i.e. RCBS, Forster, etc.)