Could for different reasons, but to name a few, are you sure your meter or whatever your method your using is accurate? Also, some soils aren't ph stable, and added to that, with only using a small cup as a container, having less soil means the pH itself in the soil can shift 'faster' because of it being less mass, it dries quicker. There is a lot of controversy based on whether pH is affected by soil wetting and drying but I tend to think it has a lot to do with it especially if there isn't a large amount of soil because it would make any shift more abrupt and drastic because of less time being needed for it to happen. Keep us updated on how everything turns and keep asking around here if you need more help with anything.
Mango
