Hmmm, what I think you're seeing is actually satoshidice fudging the payout multiplier and to make nice even numbers as well as you end up compounding some differences because the payout multiplier is rounded to three decimal places. I think maybe the less than 8000 bet should be 7.980x or something, but 8x looks a whole lot better.
I thought that computing the expected values of each bet would illuminate this but it's just confusing me more. Since the house edge is supposed to be 1.9% all of the EV's should all be around
0.981 but they're not, so unless I'm missing something satoshidice is fudging some numbers, making their house edge a little fatter than they're letting on 0.976 since .005 is returned with losing bets. Then the question is why are there bets that seem to pay out more?
Feel free to double check my math, also the odds of winning are only truncated for display but that math is based off of calculating the actual odds:
Bet Odds Payout Expected Value
1 0.000015259 64000.000 0.9765625000
2 0.000030518 31981.573 0.9760001526
4 0.000061035 15990.789 0.9760003052
8 0.000122070 7995.397 0.9760006104
16 0.000244141 3997.701 0.9760012207
32 0.000488281 1998.853 0.9760024414
64 0.000976563 999.429 0.9760048828
128 0.001953125 499.717 0.9760097656
256 0.003906250 249.861 0.9760195313
512 0.007812500 124.933 0.9760390625
1000 0.015258789 63.968 0.9760742188
1500 0.022888184 42.647 0.9761123657
2000 0.030517578 31.987 0.9761657715
3000 0.045776367 21.326 0.9762268066
4000 0.061035156 15.996 0.9763183594
6000 0.091552734 10.666 0.9765014648
8000 0.122070313 8.000 0.9765625000
12000 0.183105469 5.335 0.9768676758
16000 0.244140625 4.003 0.9772949219
24000 0.366210938 2.670 0.9777832031
32000 0.488281250 2.004 0.9785156250
32768 0.500000000 1.957 0.9785000000
48000 0.732421875 1.338 0.9799804688
52000 0.793457031 1.235 0.9799194336
56000 0.854492188 1.147 0.9801025391
60000 0.915527344 1.071 0.9805297852
64000 0.976562500 1.004 0.9804687500
There is an interesting correlation though , the less than 60000 bet is the highest EV and less than 8000 is a bit lower with the less than 2 bet being the lowest. What happens with your strategy if you try a martingale on the less than 2 (ignoring mins and max bets for hypothetical purposes)?