Why is the groundwater table in Beijing Plain the lowest in July? Isn't there a lot of precipitation in July?

The depth of groundwater level refers to the distance between the surface and groundwater level. As long as it is supplemented by precipitation and surface water infiltration through the seepage zone.

As can be seen from the depth on the left side of the figure, the deeper the groundwater level is, the deeper it is. That is, the more precipitation infiltration, the more precipitation. So Beijing has the most precipitation in July. This is also the case in the picture!

You've got the unit increase in the wrong direction in this picture.