It's great that you're experimenting with this, and observing the outcome. The results you describe sound right, as the PA 28, though an excellent wing, has a stabilator, which is a little less than ideal for optimum low speed pitch control. Several times, I have gotten PA-28's "hung" while experimenting with short/soft field techniques.
The stabiliator can be stalled, as it's only way of changing to create more lift (downward) is to change angle of incidence/attack. Whereas the traditional stabilizer & elevator changes camber, making it a bit more stall resistant, and the fixed (or nearly so for many C 180 series) stabilizer is more stall resistant. The early C 177's suffered from this, and that's why 177's have upside down slots in the stabilator.
I found the PA 28 to be extremely technique sensative in short/soft field takeoffs. Too much pitch up control, or at too low a speed, and the drag from the stabilator overcomes any advantage you might have had in getting airborne early. In one case, my buddy got his Arrow off the ground at too low a speed, and it would not climb, or accelerate, it was just hung in ground effect, and not going to clear the obsticles at the end of the 3000 foot runway. Once I knew we were committed, I raised the gear, and the drag reduction was enough to get us climbing a bit. In another case, I was taking off a frozen lake, and distance was not an issue, so I just let it keep going, 'till it climbed - it was a long way!
Takeoff performance data gathering takes dozens of takeoffs, with extreme precision, and the data averaged, and corrected for the local conditions. More than I'm sure you plan on. The determination of "50 feet" is more than just an altimeter - it's not precise enough.
Within the techniques in the POH, and your skills, keep experimenting. When you have a 1974 or later C 172, you'll see a much more pronounced difference resulting from the differing techniques, the PA 28 is just not the best plane to show the differences in contrast.
In any case, your challenging yourself, and making these observations will really increase your skills - excellent!