The angle of the sun can really play games with chronographs.Since you confirmed your Magnetospeed is running nearly the same,I say your throat is the issue.The closer the bullet is to the lands the higher the pressure and velocity your likely to see.Add a little erosion build up and it's going to increase even more.I have a 300WSM.I had to load 165gr bullets like they were 180gr bullets.My velocity was what I should have been getting,but I had to load about two grains less powder than what the books showed for that weight bullet.I fire lapped that barrel and it did change things a lot.My loads dropped about 100fps.For the first time with that rifle I could load like the books showed for the specific weight bullet.Seating a little deeper can make a big difference,especially if your at the max in your load.This will explain what may be happening. https://www.hornady.com/team-hornady/ballistic-calculators/ballistic-resources/internal-ballistics

~Molɔ̀ːn Labé Skýla~
As Bob Hagel would say"You should not use a rifle that will kill an animal when everything goes right; you should use one that will do the job when everything goes wrong."Good words of wisdom...............