In recent years, surface coatings or treatments have been used increasingly on paper to improve their runnability or otherwise fit them for use. Sometimes this treatment or coating (which may be undetectable to the eye and hand) has a pH that is much higher or lower than the paper it is applied to.
This makes spot tests misleading, and can lead to a lot of anguish. A distributor or sales agent, for instance, may know they are selling alkaline paper, but if the customer is using a pH pen that appears to tell a different story, it may be hard to convince the customer that the paper is really alkaline. (Printers, however, and others who might be more concerned about surface characteristics than about permanence, would not be so disturbed about the discrepancy.)
In order to learn whether a paper is really alkaline nowadays, you have to accept the alternatives described in the new ANSI standard for paper permanence, Z39.48-1992: either tear the paper to expose the inner surface, and do a spot test with a pH indicator such as chlorophenol red, or accept the manufacturer's certification that the core paper is alkaline and was made at an alkaline pH. (Reprinted from the Sept. Alkaline Paper Advocate, with revisions.)