For many years the center of the seat post clamp was usually 1 inch/25 mm behind the center line of the post pillar. That is called a 25 mm setback. On the more or less standard frame it helped to get the normal person's foot in a good position over the pedal for the riding style of the time. Nowadays some folks like to ride more forward and/or buy frames with a slacker seat tube angle. To get the saddle position they want fore-aft and still have their saddle close to centered on the clamp (which BTW is NOT required at all), they need a post that is not set back as much. Hence the rise in popularity of the zero setback seat post.