When I got a seat post that let me move it a little bit further back the saddle seemed to work much better. This very well have been an indication that my knees were too far forward and moving the saddle back helped correct that.
In your case it sounds like you realized that you were sitting on the back of the saddle. Once you got the setback seatpost you were no longer sitting on the back of your saddle and were properly positioned on the saddle.
Some people run into the short rail issues during bike fitting's when they're properly positioned on the saddle but can't get the seat back far enough to drop a plumb from the tip of the knee to the end of the crank arm, or soft notch of knee through pedal axle(depending on which method you use).