Hey mate, the PH fluctuations will be due to different nutrient intake.
Heres a really goo and I depth explanation as to how the nutrient imbalance can change the PH and what elements are causing a drop/increase in PH. It's super in depth it's taking me a while to get my head around it. The but you want to look at is nearer the bottom. It may not interest everyone 😝
https://extension.uga.edu/publications/detail.html?number=B1256&title=Essential%20pH%20Management%20in%20Greenhouse%20Crops:%20pH%20and%20Plant%20Nutrition
But the bottom line is some of your plants are a tad out of the ideal range 6.0-7.0. You can adjust this by flushing with PH'd water. Then add your feed after.
Hope this helps 🙂