Elements Not Summed Up Correctly

I always play Boss Rush everyday especially level 3 & 4 since I can get level 5 elements there which I need to upgrade my armors to maximum. What I noticed is that when you finished the fight and showed you how many elements you’ve got, many times it’s not adding up correctly. You can look first at how many elements you have already and then play and when you finish a fight, verify it. Mine happens many times when I got 5 up to 7 elements in 1 fight. Overjoyed only to learn afterwards that I only got 1 or 2 elements which make me sick!