Essentially this is likely due to heat rises and evaporation of any water takes place faster in warmer temps than colder temps.

A rule of thumb is that humidity can differ from one area of the home to another. Typically those differences should be in the 1%-5% range. If an area of the home is higher than that it points to problems. Those problems could be due to wide range of issues from leaky duct work bringing in humid air from outside if the HVAC system is in Attic or the duct work is. Infiltration in some form from humid air outside, evaporation of a water source some where in the structure. etc and so on.

Other issues can be shower related activity. Long hot steamy showers by a person or two daily in a warmer place in the home can easily increase humidity levels on that floor of a structure.

click to enlarge.

While this picture is of the great outdoors, the same effect can take place in a building.

I service the Katy, Texas area.

What you are describing is somewhat unusual. I assume you are taking relative humidity measurements. How are you measuring the humidity and what is the temperature of the three levels?

It's not uncommon for relative humidity to vary as a function of height within a structure. The relative humidity on the 2nd floor of my home (in a coastal Virginia climate) is often higher than it is on the 1st floor level. Moist air is less dense than dry air and is more buoyant.

I've found dehumidifiers to be an effective means of controlling relative humidity during shoulder seasons--when air conditioning is not being called for-- and thus the air conditioner is not performing any dehumidification.