Funny i have had the same issue for a long time now myself. I have determined that it is mainly when i buy a new bag of grain or a different malt. There seems to be a variance in PH between new bags/malts however not by much. I use to use EZ water and you can change the distilled water PH value of each grain of which i did for a while and it's just a big pain in the rump. Brunwater is by far the best i think.
I recall some of your previous post and we seem to have the same processes, Funny. Anyway i tend to always shoot a little higher (5.6 room temp) and it works. I have become better at adjusting on the fly and its been fun. good luck wish i could help.