Why do Baytown floors read damp along the outside walls?

Because the ground under the slab is gumbo, and gumbo holds water. After rains, the clay at the slab's edges stays saturated long after the yard looks dry, and moisture argues upward at the perimeter, exactly where baseboards and flooring edges report it first. Readings there have to be compared against the house's own dry interior standard, and the corrections are usually drainage and grading, where the downspouts discharge and how the lot sheds water, rather than anything exotic inside the walls.