In my opinion it is somewhat the mixers fault.The scales they use to mix the paint can tell the difference in two drops of tinter. On the other hand in my experince I too don't use two different mix batches. There are just too many other varyables ,coverage,air presure,humidity,temperature and how heavy a guy lays it on. So a guy doesn't need to have to think about batch mixes to. I've even painted another part a day later using the same can of paint and had a mismatch.You have got to hold your face just right and pray [ a bunch] Good Luck and have FUN