Not all bags are created equal. Unless a bag is labeled as absolute or high efficiency, I would assume it's nominal. From this website:
"Absolute vs. Nominal Micron Ratings
The terms Absolute and Nominal are often associated with micron ratings given to liquid filter bags and cartridges.
As the name suggests, absolute conjures up a notion of 100% pure filtration. Absolute rated filter bags and cartridges, contrary to their name, are not absolute. They are generally gauged by the percentage of contaminate particles they are capable of removing in perfect laboratory conditions. For example, a one micron absolute rated filter bag will remove between 99.5% and 99.9% of laboratory "challenge particles" one micron or larger. They are not 100%. For this reason, we refer to this type of filter bag as "High Efficiency", stating the corresponding media efficiencies from independent laboratory test results.
On the other hand, nominal micron ratings are average or general in nature. Their efficiencies can fall anywhere between 50% and 95% with the norm being around 80% efficiency. That is to say, a one micron nominally rated filter bag will remove approximately 80% of "challenge particles" one micron or larger in perfect laboratory conditions. The terms Absolute (High Efficiency) and Nominal should be used as a guideline only when selecting the best filter bag or cartridge for an application. Unless stated as "High Efficiency", micron ratings used in our nomenclature are nominal."
I buy my 1 and 5 micron "nominal" bags off of eBay, but use the material only as a pre-filter.