What happens with soil Ph is that it mixes with the liquids you give it to form a new Ph. Your soil is probably acidic and you watered with plain water. The more plain water is in the soil and the sooner after watering you test it, the closer the soil Ph will be to the liquid's Ph. For example, if the soil is Ph 6.7 and you soak it with plain water, there may not be enough acidifiers to lower the water's Ph. When you test the wet soil, you get a reading of 7.0 because that's the water's Ph. As the substances in the soil slowly dissolve into the water, the Ph will slowly change because more acidifiers are dissolving into the liquid. Also, as the water dries out, there will be less Ph 7 liquid compared to how much Ph 6.7 soil there is, causing the Ph to fall further.
The Ph of your soil is the reading furthest from the Ph of the water you give it. If you water with plain, Ph 7 liquid and the soil tests more acidic each day as it dries out, the soil Ph will be the most acidic reading you get.
The optimum Ph level for plain soil is around 6.5. Ph 6 is used with coco coir, hydro and a few other, less common mediums.