This is the most over-claimed word in luggage, and the physical reason is simple enough to check yourself.

A coated fabric is not a waterproof bag

Coating stops water passing through the weave. That is genuinely useful and it is one part of the problem.

The other parts are the closure and the seams, and coating does nothing about either.

A zip is a row of gaps

Even a water-resistant zip is a mechanical closure with a path through it. Under rain, spray or a bag lying on a wet surface, that is where water arrives first.

Storm flaps help. They are not a seal.

Every seam is thousands of holes

Sewing a bag means punching a needle through the panel several times per centimetre. On a coated fabric you have just perforated the thing you paid to coat.

Taped seams cover those holes and are a real improvement. Welded seams do not create them in the first place.

What genuine waterproofing looks like

Welded seams, no zip, a roll-top closure folded down and clipped, and a heavy laminate fabric. That is why dry bags look the way they do — the appearance is a consequence of the requirement.

It is also less convenient. A roll-top is slower to open and it wastes some volume every time you close it.

Choosing honestly

If the bag lives on a boat, a bike or a site, buy the welded roll-top. If it crosses a wet car park occasionally, buy the coated water-resistant version and save the money — you will find the roll-top annoying for no benefit.

What should never happen is the word waterproof appearing on a sewn, zipped bag. If a supplier is loose about that, be careful about the rest of the specification.