When bending or "moving" metal you have to take into consideration the spring-back. When moving shoulders back or for that matter sizing necks, you have to exceed the "modulus of elasticity". The point you displace metal and it doesn't return to it's original position when pressure is released.
This "Modulus" in brass depends greatly on the hardness of the brass. If you want uniform shoulder "bump" or neck sizing you just about have to anneal. For me i consider it a necessity. LC brass is a little tougher than let's say winchester or Remington (R-P headstamp). If you don't anneal yet, get a heavy deep impact socket that the brass just slips into. It should stop with just the shoulder area sticking out with just a portion of case sidewall exposed. With a 1/4 drive adapter that fits in a cordless drill and fits the socket, chuck it up in a drill. Using a butane torch, spin the brass in the flame until it shows a fairly pronounced blue color without glowing red. It takes me about 4 seconds in the flame to anneal a case.
Drop the hot case in a metal bowl and you don't need to quench it in water. Doesn't help the annealing and just makes wet cases.
Then try bumping shoulders and you should see some consistent readings with your mic. Last item of importance, make sure your press is pushing he die the same amount. If in doubt use shim washers to adjust setback rather than backing out or screwing in die. Can also use a Redding Competition Die Set which has shell holders that are are ground with .002" increments so you can lower the case in same amounts. Start with the "+ .010" shell holder and size case. Check shoulder setback. If not where you want it, move to +.008" and repeat. As an example I find that using the .006" shell holder gives me a perfect shoulder setback on my annealed cases with the die screwed in to the point where the press "cams over". By adjusting the die to his point there is no slack in the press and I'm putting the same pressure every time on the case.