>Results vary a lot with only minor changes to any of the input values:

result: 7.046408425590759alpha:2.4603574070439738 beta: 0.6170228812798457 gamma: -0.10332701324789607
>>>>result: 346.38209648523616alpha:2.4529665255733537 beta: 0.7445035661819629 gamma: 0.14693780311807225
>>>>result: 323.7233478510366alpha:2.4845233810017673 beta: 0.833801293904539 gamma: 0.5579781136510195 
>>>>result: 329.008299842608alpha:2.4764343283698595 beta: 0.8381191267246367 gamma: 0.4553249133004684 
>>>>result: 334.725548494047alpha:2.4684135113669075 beta: 0.8278849276394666 gamma: 0.348097048499672
>>>>result: 340.71391817968237alpha:2.461853051087034 beta: 0.8186382448641772 gamma: 0.2475292789778855 
>>>>result: 347.15633074038885alpha:2.4561540103590627 beta: 0.8008004917341757 gamma: 0.14678892798674084 
>>>>result: 354.4269473034584alpha:2.451044891296479 beta: 0.7906537474295814 gamma: 0.04312349358490233 
>>>>result: 1.7843130240315808alpha:2.4428582726856356 beta: 0.7780242913386914 gamma: -0.05750370999075051 
>>>>result: 9.100154926831845alpha:2.4387029626060905 beta: 0.775427620358397 gamma: -0.1583050342919901 
>>>>result: 359.885344715594alpha:2.4628581134963703 beta: 0.6254340599609843 gamma: -0.025646558918639244 
>>>>result: 4.523793658145151alpha:2.596318259831484 beta: 0.5869759139752974 gamma: -0.07331963792916502 
>>>>result: 5.763368012442853alpha:2.596818384656943 beta: 0.7196717736453192 gamma: -0.10575804385912449 
>>>>result: 12.11694797364695alpha:2.6004789464347144 beta: 0.7852986395008963 gamma: -0.20626720572883836 
>>>>result: 5.5265596587107835alpha:2.604506988938409 beta: 0.7266541171534491 gamma: -0.10381731352376557
>>>>result: 14.73266921491841alpha:2.602183666809788 beta: 0.7347126347609391 gamma: -0.22932036683207827 
>>>

>>>> Haven't dug into it yet....
>>>>
>>>
>>>Update - I was wrong - I forgot the signs of the gamma values
>>
>>OK. I updated the relevant gamma values, ran the C# version and a javascript version using the same data. The results were identical but still skip around more than is warranted :-{
>
>
>If you only convert the alpha to degrees - what about that ?
>

>140.96809551737
>140.544629202222
>142.352703832975
>141.889235256908
>141.429676294392
>141.053789608692
>140.727258627706
>140.43452766839
>139.965468973505
>139.727387243379
>141.111375442894
>148.758078561094
>148.786733602823
>148.996468343336
>149.227258178497
>149.094141626078
>

>
>http://w3c.github.io/deviceorientation/spec-source-orientation.html
>

>A device lying flat on a horizontal surface with the top of the screen pointing West has the following orientation:
>      {alpha: 90,
>       beta: 0,
>       gamma: 0};
>

>
>You may have to subtract 90 if the north is zero degrees

Just an update: turns out that the root problem was indeed hardware on my Nexus. Got another one to try and that works flawlessly (using just alpha data) - next thing is to try again with the function that adjusts for beta/gamma.