I have redone this test with modifications and came up with even more confusing results, the exact opposite of what I was expecting !!
To summarise my initial problem: I was getting a lower gamma (0.42 compared to the target 0.5) and I though this was due to the target I photographed introducing too much flare (in spite of me intending that it wouldn't - I suspect I wasn't thinking straight). The initial target was a matt cream coloured wall, but I only framed about a small 10% of the centre of the wall and in hindsight then I think the rest of the wall might have contributed to flare both in and out of the frame thus reducing my contrast. So to rule that cause in or out I repeated this test and found something even more strange which I need help understanding.
I decided to find a target with much less opportunity for creating flare. So I draped a white towel folded in half in front of a dark green hedge. The towel in the centre filled about 30% of the frame. I exposed the towel to sit on Zones III through VII. The hedge and the area out of the frame in front of the camera were a few stops darker than the towel. It was in shade and I couldn't see the sun.
So with another roll of film I now separately shot two targets. The first Five frames had this new towel target and another 5 frames had the original cream wall target. I fully expected the gamma calculated with the towel shots would be higher than the wall shots. It was in fact the exact opposite . Towel gamma=0.46, wall gamma=0.55 . The towel gamma should be equal to and probably higher than the wall gamma. (The target gamma in both cases was 0.6)
So what can possibly explain this difference ? Experimental error ?
Here are some misc notes in case you have read this far and might be thinking about an explanation: It turned out by chance that the exposure reading of each target was within 1/4 of a stop of each other (the cream wall was in a slightly higher light level). Accordingly I used the same shutter speed for both targets and simply varied the aperture from f5.6 to f22 in steps of 1 stop. I also took the opportunity to expose and develop for N+1 rather than N (as per the first check roll I did). This at least proved my gamma increased by about the expected 0.1 . The 5 data points from each target all lay quite well on the straight line portion of my HD curve. This rules out any light level or shutter speed variations. I double checked my densitometer calibration before and after measuring the densities. Also because both targets were on the same roll of film this rules out the possibility of dev strength/temp/timing errors. I also used a second spot meter this time to double check the exposure readings.