Why does the CDL manual warn drivers not to simply trust posted bridge and overpass clearance heights?

49 CFR 383.111(a)(10); AAMVA CDL Manual (nhcdm.pdf) Section 2.7.4, p. 2-17, 'Space Overhead'

Overhead strikes are a common and expensive cause of commercial vehicle crashes precisely because drivers treat a posted clearance sign as a guarantee rather than a starting point. Knowing that both road conditions (repaving, snow buildup) and the vehicle's own loaded state can silently change the real clearance is what separates a cautious professional driver from one who gets surprised by a bridge strike.