为什么BCH挖矿可以半小时一个块，也可以半分钟一个块？

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505430 Nov 23, 2017 8:39:01 PM 228 97941
505429 Nov 23, 2017 8:27:04 PM 65 AntMiner 30019
505428 Nov 23, 2017 8:22:58 PM 109 66406
505427 Nov 23, 2017 8:17:24 PM 379 142176
505426 Nov 23, 2017 7:59:35 PM 6 1506
505425 Nov 23, 2017 7:59:05 PM 209 149513
505424 Nov 23, 2017 7:50:58 PM 421 225594
505423 Nov 23, 2017 7:29:46 PM 5 1786
505422 Nov 23, 2017 7:29:28 PM 58 41249
505421 Nov 23, 2017 7:26:36 PM 18 6256
505420 Nov 23, 2017 7:26:28 PM 23 8989
505419 Nov 23, 2017 7:24:19 PM 72 AntMiner 33091
505418 Nov 23, 2017 7:21:24 PM 342 151255
505417 Nov 23, 2017 7:03:24 PM 14 15202
505416 Nov 23, 2017 7:02:54 PM 423 259721
505415 Nov 23, 2017 6:42:54 PM 143 126555
505414 Nov 23, 2017 6:35:54 PM 309 149884
505413 Nov 23, 2017 6:24:25 PM 34 11915
505412 Nov 23, 2017 6:19:40 PM 286 AntMiner 177984
505411 Nov 23, 2017 6:06:01 PM 152 AntMiner 58134
505410 Nov 23, 2017 6:02:30 PM 806 425428
505409 Nov 23, 2017 5:22:59 PM 206 75514
505408 Nov 23, 2017 5:17:50 PM 507 239024
505407 Nov 23, 2017 5:00:03 PM 1115 423610
505406 Nov 23, 2017 4:27:30 PM 666 297812

不是说升级了稳定多了么，为什么还是这么抽风？