I am not entirely sure what is going on but it appears to be a carbonate or temporary hardness/acid binding issue. When CH decreases, pH increases. If there is free/radical carbon dioxide, CH will increase and pH decreases.
Testing water once for dynamic parameters can be a waste of money. You're taking a snapshot of something that is constantly changing.
A CH/GH issue fits the limited data but the stratification is somewhat befuddling.