Read the How it worked part of the kata's description, you can't change horses or hand over the mail bag in the middle of nowhere, only at stations.

Distance          Rider number
19 + 67 = 86      1
34 + 20 + 41 = 95 2
85                3
47                4
65 + 28 = 93      5
54                6
69                7
56 + 6            8
71                9
60                10
88                11
15 + 24           12
77                13
64                14
94                15
98                16
65 + 1            17
42 + 28 + 23 = 93 18
81                19
67                20
35 + 16 = 51      21
85 + 2 = 87       22
50 + 4 + 43 = 97  23

And that's how each rider doesn't ride more than 100 miles each in that case.

In music theory, this is called an inverse interval.

I agree

1. First array: 5 + 5 = 10

   numbers = { 799, 379, 744, 394, 158, 523, 320, 270, 763, 990, 966, 793, 823, 198, 80,
   608, 956, 69, 132, 961, 293, 652, 444, 158, 824, 402, 640, 963, 54, 597, 468, 572,
   695, 212, 965, 852, 453, **5**, 840, 934, 713, 525, 446, 536, 722, 525, 862, 397, 593, 
   993, 77, 604, 645, 520, 481, 188, 640, 839, 152, 413, 436, 619, 984, 131, 549, 668,
   701, 721, 391, 541, 655, 104, 66, 102, 359, 506, 345, 221, 621, 657, 215, 417, 979,
   578, 936, 460, 766, 296, 299, 636, 708, 454, 973, 692, 584, 241, 79, **5**}
   

2. Second array: 10 + 10 = 20

   numbers = {592, 979, 74, 623, 464, 428, 757, 50, 771, 597, 951, 294, 586, 289, 440,
   685, 111, 205, 175, 193, 768, 577, 621, 302, 429, 455, 271, 59, 563, **10**, 295, 873,
   989, 88, 496, 453, 234, 971, 221, 723, 568, 173, 736, 154, 180, 176, 557, **10**, 100,
   732, 920, 586, 28, 541, 887, 176, 996, 158, 234, 559, 168, 247, 432, 875, 334, 646,
   47, 568, 617, 268, 10, 903, 159, 745, 58, 57, 921, 614, 66, 739, 346, 985, 325, 374, 527}