If only I could answer that question !!! You have just experienced the tried and true ... " Work up the load for your gun , and, Every gun shoots different.. "
There is a excellent article on "Why doesn't my gun shoot at the Reloading Manuals Velocity ? " ... while that seems to have no bearing on your question... it really does help show that in shooting ... not all is what it seems. Note the velocity difference between the same type and make of the guns....http://www.leverguns.com/articles/ballisticians.htm