The two mistakes
Insufficient profile — polished or under-blasted steel — gives the coating too little mechanical key: adhesion drops, and immersion service exposes the deficiency first. Excessive profile relative to the specified film thickness leaves peaks protruding near or through the coating: those peaks become rust spots on a coating that measured "in spec" on average. The rule of thumb the industry works to: film thickness must comfortably cover the measured profile.
How it is measured
Field measurement uses replica tape (pressed onto the surface and read with a micrometer) or depth micrometers with pointed anvils; your specification's cited method governs. The profile requirement comes from the coating manufacturer's datasheet for the specific product — not from a universal number — and abrasive selection (size, shape, hardness) is how the applicator hits it.
Profile is not cleanliness
Profile and cleanliness are separate requirements measured separately: a surface can be blasted to a beautiful near-white cleanliness with the wrong profile, or carry a perfect profile under invisible salt contamination. A complete surface preparation clause specifies both — plus the salt limits — and the QC records prove each.