When titrated against a standard base, by experts, under very carefully controlled conditions, they found that it neutralized a slightly greater amount of base. Probably, it contains a slight impurity of another acid, another compound, or a breakdown product, I'd guess phthalic acid. So when you use this batch of potassium hydrogen phthalate, you'll use that purity, and get the correct titer value for your base.
This happens often, sometimes a trace impurity has an effect on the bulk properties much greater than it's concentration. In the example above, phthalic acid is a diprotic acid, so it's effect, on a base neutralization, is twice what it's concentration is. As another example, purified enzymes, or hormones may have activities a fraction of a percent above what their purity suggests, that could be a trace of another, more active hormone, or the removal of an inhibitor.