The answer is previous over-processing. Milady states directly that hair with a high porosity level is considered overly porous and is usually the result of previous overprocessing.
The book describes hair in this overly porous condition as dry, damaged, and fragile — it absorbs liquids quickly, but the damaged, open cuticle scales can actually make it harder to retain haircoloring pigments evenly. Internal disorders, poor nutrition, and hormonal changes are real factors covered elsewhere in the chapter, just not the one tied to high porosity specifically.
Milady Standard Barbering, 6th ed., Ch. 10 (Properties and Disorders of the Hair and Scalp), p. 267: "Hair with a high porosity level is considered overly porous and is usually the result of previous overprocessing."
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