I have had 5!! E-M10s.. All of them with a very tight lens mount (3 of them had some scratches, so returned them because of that).
I get a sore thumb after removing and placing a few lenses. All my lenses (except the 40-150) are tight: 45mm, 12mm, 9-18mm, 20mm (<-just a little less tight).
Love this camera but this is annoying (on my ep1/epl2 it was so easy to mount a lens) just got to live with it unfortunately..
All mass produced items have tolerances..maybe your mount at min and lens base at max..simple as that...anyway when you use third party adapters they have serious diiferences..like someone said better tight than slack and light allowed to pass. I know this from experiance.