Actually the low watts and the below intel doc may be close to making sense with the 5C gradient we measure...if the chart below from intel is correct, which implies the gradient from core to casing increases (doubles) when removing the heatsink.
http://www.intel.com/support/process.../CS-011039.htm
Tjmax-tcasemax = theta (jc) x TDP
Solving for theta with Tjmax 100 = .43 C/W
If the thermal resistance from core to casing (theta) does increase/double without a heatsink as intel states, then if an actual 1.6-2.5C gradient does exist we would measure it as 3.2 to 5C with heatsink removed.
Or, stated another way, instead of actual gradient (heatsink on) = .43 X TDP, gradient we measure with heatsink off = .86 X TDP. At idle, extended halt intel lists 8W, deep sleep 6w, and at idle undervolted, underclocked probably more in 6W or less range...clearly this will be variable cpu to cpu and some are testing even lower. But at 6W (max?), the gradient would be 2.5C, we would measure 5C, assuming the intel doc is correct. If true, that would suggest tjmax is 100, when we measure the IHS with heatsink off we would measure 95C, if we could measure the IHS with heatsink on we would measure 97.5, which would make sense with the actual ~2C gradient. But other than a calibrated thermocouple embedded in IHS...dont know how to test that.