I didnt read all the replies but here are my thoughts after reading your question. were your shells sitting in the sunlight or at least out in the heat as you were shooting I have found that the warmer my ammo gets the higher my velocity also as my chamber heats up same things happens it slowly creeps up. you should try shooting a couple shells each day instead of all at once and try to keep the variables the same from day to day and see what you get.
" Real elk guns start with the number 3 or bigger and blow two holes, one in and one out." - My Dad