Something to consider here is that the current army of Slayers were activated by Willow's spell, not Called in the regular way. Possibly, this would be somewhat comparable to picking a fruit before it matures and ripens naturally as opposed to the "Vine/tree ripened" variety. I'm not saying that a Slayer is like a Macintosh apple, necessarily, but if being a Potential is in fact genetic, and Calling somehow activates the Slayer properties, Willow's spell might not have completely fulfilled the process, resulting in "pseudo-Slayers," demon fighters who are almost, but not quite, the real thing.
Or, to borrow Buffy's metaphor, they were taken out of the oven before their cookies were fully baked.