I had not previously read killerbmotorsports comments, but the 45 degree entry into the oil pan makes perfect sense. I have even made a geometric sketch of a dipstick entering the oil surface at a 45 deg angle, and that can easily account for the difference in readings, front to back.
As an aside, I feel that we are needlessly obsessing of the possible difference of a few ounces of oil, one way or the other. IMHO, if you measure anywhere between the 'L' and 'F' marks, with or without oil cap and/or o-ring, you are fine.
Thanks for straightening me out.................. Frank