You will get different readings at the top of the soil as compared to the bottom. The difference are cause by differences in the root development and roots do tend to grow down and towards bottom where more water may have been accumulating or staying the most moist in a pot. More root activity and metabolizing does mean more ion and anion exchanges and possibly more acid.
5.0 is way too acid, you don’t want it below 5.5 and preferably not below 6.0 for soil. There is a bit of a debate about the perfect range for cannabis in soil, I say the best range is 6.0 - 6.5 in soil with a 6.2 as ideal, now – after a bunch of new studying and information I’ve collected, however the older traditional thought is 6.5 - 7.0 for soil. Also it is thought very acidic conditions increase the amount of males and or hermaphrodites when grown from regular seed, and so 6.5 -7.0 is still likely best in the beginning for when growing a bunch of regular seeds. And 5.5 - 6.0 in hydro with 5.8 ideal. Soil can go up to as high as 7.0 or even a bit higher, certainly no higher than 8 and I’d start to get worried if it stayed there for a long time. But even staying at 7 in a good organic composted soil you likely would not have many problems. In hydro I don’t worry so much about it staying very stable day to day, as it goes above or below the 5.8 the plant will take advantage and take different nutrients that become more available at different levels, as long as you don’t go below the 5.5 or above 6.5, you normally won’t have much to worry about in hydro.
How are you getting the two different readings? A PH probe stabbed in the top of the container’s soil and a PH pen testing the runoff water?
You might need to be adding some of the ag lime PH up but I would add very very little to start with, especially if the plants look very healthy, if it ain’t broke, don’t fix it. Right?