Yeah, the material is waterproof not water resistant. Big difference there.
For those wondering why we use a non-waterproof side zipper in an otherwise waterproof-able bag....
A truly GOOD waterproof zipper that is durable is extremely expensive. Think drysuit scuba equipment, maybe kayak skirt zipper.
Most "waterproof" zippers are merely urethane coated zippers that are water resistant. The downfall here is that the urethane coating wears down and starts to separate from the zipper. These zippers wear out much faster than a standard zip.
What we use is a standard zipper, but in a vertical orientation, and we sew it in so that it is highly water resistant. In our testing water tended to enter through the stitch holes around the zipper instead of through the zipper teeth. Seam seal those holes and you've got a very water resistant (rain, not submerged or whitewater rafting) pack bag.
Get a roll top pack bag with no zipper and then seam seal it and it may be truly waterproof.
