They mentioned 97-99 percent efficiency, but they didn't mention the microns on which this efficiency was tested.
As an example, a filter is 99% efficient at 20 microns, but its efficiency will not be the same at 15 or 10 microns. So this percentage is vague until or unless microns are mentioned.
Engineering Explained did an interesting experiment comparing OEM, K&N and aftermarket filters. The results were interesting