I am not familiar with iDip tests but here is what is happening: the PO4 reading is for Orthophosphate, or Phosphate, and the P reading is for Phosphorus.
To convert from units of orthophosphate (PO43-) to phosphorus (PO4-P), multiply the value by 0.3261. To convert from phosphorus (PO4-P) to orthophosphate (PO43-) divide the value by 0.3261.
By using the simple formulas above, you'll see that your iDip just makes some calculations.
In short, if you're looking to get your phosphate reading, and I guess that is what you're after, just use the PO4 reading.
Hope this helps!