[argyllcms] Gamma measurement discrepancy.

Hello,
I'm a beginner trying to set up an LCD monitor (Samsung 245T) using Argyll and a Spyder 2, on Linux. I'm puzzled by the difference between the gamma value reported after calibration by dispcal -r, and the gamma values measured using various online squinty test patterns. I wonder if anyone can explain the differences that I'm seeing, and perhaps tell me if I'm doing something wrong.
Since I still have a few non-colour-managed applications, I'd like first to set up the monitor to be close to an sRGB standard through calibration alone: 

dispcal -yl -gs -t6500 -b150 -v2 -qh cal_srgb_hq

After adjusting only the monitor controls, dispcal -yl -r reports:

Current calibration response:
Black level = 0.42 cd/m^2
White level = 149.58 cd/m^2
Aprox. gamma = 1.93
Contrast ratio = 352:1
White chromaticity coordinates 0.3120, 0.3297
White    Correlated Color Temperature = 6536K, DE to locus =  5.5
White Correlated Daylight Temperature = 6535K, DE to locus =  1.1
White        Visual Color Temperature = 6335K, DE to locus =  5.3
White     Visual Daylight Temperature = 6497K, DE to locus =  1.1
At the same time, I estimate a gamma of about 2.9, using charts such as those at <http://www.lagom.nl/lcd-test/gamma_calibration.php>, or <http://www.normankoren.com/makingfineprints1A.html#gammachart>.
Then I do the calibration with dispcal. It goes OK, as far as I can tell. The output file and stdout capture can be found at: 
<http://www.morphet.org.uk/downloads/misc/cal_srgb_hq.cal> and
<http://www.morphet.org.uk/downloads/misc/cal_srgb_hq.txt>.

So I load the calibration using dispwin, and run dispcal -r again:

Current calibration response:
Black level = 0.42 cd/m^2
White level = 147.70 cd/m^2
Aprox. gamma = 2.10
Contrast ratio = 348:1
White chromaticity coordinates 0.3111, 0.3291
White    Correlated Color Temperature = 6589K, DE to locus =  5.6
White Correlated Daylight Temperature = 6588K, DE to locus =  1.3
White        Visual Color Temperature = 6379K, DE to locus =  5.4
White     Visual Daylight Temperature = 6544K, DE to locus =  1.2
Checking again with the online charts, I now estimate the gamma to be about 2.6.
So, my question is, why the big difference in gamma figures, and which ones should I believe? I appreciate that dispcal is only reporting an approximate value, and that there is probably also some error in my reading of the online charts, but surely that can't account for so much difference. And why did the dispcal gamma increase when the calibration was loaded, yet the chart gamma seemed to fall?
The calibration does seem to have improved the way things look on my monitor, but can I trust it? I feel like the proverbial man with two clocks. 

Steve.

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