Suuntos and CNS clock

[watboy]

I did my PADI tec deep course a few weeks back. Firstly, yes, I dove cut tables, but for fun I left my nitrox recreational dive computers on. I dove 2, a D6 and a Vyper Air. On one of my training dives to 40m for 20mins, I had air in my twinsets and carried 100% o2 for deco starting at 6m, and entered the gas switch on both my suuntos (21% and 99%). I don't remember exactly, but I recall calculating on tables that my CNS exposure was relatively low, under 30%, it was my first dive of the day. I was on o2 for 8mins. My Vyper air however, reported hitting a CNS of 78%. My D6 gave me a whopping 135%. I would think that the CNS exposure calculation to be a relatively straight forward one and don't see how my suunto's varied so greatly from each other, or that far off from my tables. Anyone else run into something similar? Both suuntos gave the same depth reading and the same NDL and deco times (again, i dove tables, just curious how the suunto's would react to a deco dive). I switched both my suuntos on to Gauge mode for the remainder of my tec course.