Could someone explain what the heck I'm looking at here?  Why the "?"s, and why the zero payments for a lot of shifts?

Completed   Duration   Difficulty   Pay/Hour at 1 TH/s   Total Thps   Your Ghps   Your work
2014-10-14 12:23   3h 14m   35,002,482,026   ?   1,292.4   1,887.9   5,128,256
2014-10-14 09:09   3h 11m   35,002,482,026   ?   1,313.7   1,934.0   5,162,304
2014-10-14 05:57   3h 09m   35,002,482,026   ?   1,322.9   1,908.9   5,058,240
2014-10-14 02:47   3h 14m   35,002,482,026   ?   1,285.7   1,810.6   4,931,904
2014-10-13 23:32   3h 17m   35,002,482,026   ?   1,270.1   917.9   2,534,532
2014-10-13 20:14   3h 13m   35,002,482,026   ?   1,294.7   422.4   1,142,400
2014-10-13 17:00   3h 14m   35,002,482,026   ?   1,293.7   1,768.8   4,799,488
2014-10-13 13:45   3h 16m   35,002,482,026   ?   1,281.6   1,924.8   5,274,176
2014-10-13 10:28   3h 14m   35,002,482,026   ?   1,291.7   1,956.5   5,305,088
2014-10-13 07:14   3h 12m   35,002,482,026   ?   1,305.0   2,123.9   5,698,752
2014-10-13 04:01   3h 10m   35,002,482,026   0.00000000   1,320.9   2,050.5   5,448,896
2014-10-13 00:50   3h 10m   35,002,482,026   0.00000000   1,318.5   2,110.8   5,606,464
2014-10-12 21:40   3h 10m   35,002,482,026   0.00000000   1,318.1   1,622.0   4,307,568
2014-10-12 18:29   3h 08m   35,002,482,026   0.00000000   1,332.6   2,088.6   5,488,704
2014-10-12 15:20   3h 11m   35,002,482,026   0.00000000   1,310.5   2,104.7   5,625,740
2014-10-12 12:09   3h 13m   35,002,482,026   0.00000000   1,300.1   2,130.1   5,752,512
2014-10-12 08:55   3h 12m   35,002,482,026   0.00059286   1,299.0   2,124.2   5,726,848
2014-10-12 05:42   3h 15m   35,002,482,026   0.00059286   1,283.1   2,056.8   5,628,480
2014-10-12 02:26   3h 15m   35,002,482,026   0.00118696   1,287.3   2,039.1   5,557,632
2014-10-11 23:10   3h 12m   35,002,482,026   0.00118696   1,302.7   1,331.0   3,577,508

Thanks