In general, the ppm quoted on a label is the detection threshold of the test used. Zero gluten is less than 5 ppm, and also less than 20 ppm (and less than 200 ppm). But you just cannot prove zero. The tests get more expensive as the threshold gets smaller.
Glutino is a well know and trusted supplier of gluten-free products. At their facilities no gluten is ever intentionally brought in. Nevertheless, they know that cross-contamination can happen at any point on the supply chain, including at the "gluten-free" plant. Someone could enter the facility with crumbs on their clothing from lunch outside. Glutino test their products, using a test which can detect 20 ppm gluten.