The progression is based on starting from a base of 1 unit and multiplying by around 10% - 12% to get the value of the next stake. Here is a version I just coded which is very similar to the Holloway progression using a percentage of 11.5% which gives approximately the same values and the same number of steps.
The error in the Holloway progression comes from rounding. You can see the same thing here: The middle column is the calculation unrounded and in the right column the numbers have been round DOWN to the nearest whole number. Notice that from step 27 (18u) to step 28 (21u) there is an increase of 3 units, but from step 28 to step 29 (23u) there is an increase of only 2 units!
Depending on how the rounding is done there will usually be some error like this, but it won't make a lot of difference. You could always change the values in the Holloway progression slightly to make the steps between values consistent.
1 1.115 1
2 1.243 1
3 1.386 1
4 1.546 1
5 1.723 1
6 1.922 1
7 2.143 2
8 2.389 2
9 2.664 2
10 2.970 2
11 3.311 3
12 3.692 3
13 4.117 4
14 4.590 4
15 5.118 5
16 5.707 5
17 6.363 6
18 7.095 7
19 7.911 7
20 8.821 8
21 9.835 9
22 10.966 10
23 12.227 12
24 13.633 13
25 15.201 15
26 16.949 16
27 18.898 18
28 21.072 21 << 3 from 18 to 21
29 23.495 23 << 2 from 21 to 23
30 26.197 26
31 29.209 29
32 32.568 32
33 36.314 36
34 40.490 40
35 45.146 45
36 50.338 50
37 56.127 56
38 62.581 62
39 69.778 69
40 77.803 77
41 86.750 86
42 96.726 96
43 107.850 107
44 120.253 120
45 134.082 134
46 149.501 149
The error in the Holloway progression comes from rounding. You can see the same thing here: The middle column is the calculation unrounded and in the right column the numbers have been round DOWN to the nearest whole number. Notice that from step 27 (18u) to step 28 (21u) there is an increase of 3 units, but from step 28 to step 29 (23u) there is an increase of only 2 units!
Depending on how the rounding is done there will usually be some error like this, but it won't make a lot of difference. You could always change the values in the Holloway progression slightly to make the steps between values consistent.
1 1.115 1
2 1.243 1
3 1.386 1
4 1.546 1
5 1.723 1
6 1.922 1
7 2.143 2
8 2.389 2
9 2.664 2
10 2.970 2
11 3.311 3
12 3.692 3
13 4.117 4
14 4.590 4
15 5.118 5
16 5.707 5
17 6.363 6
18 7.095 7
19 7.911 7
20 8.821 8
21 9.835 9
22 10.966 10
23 12.227 12
24 13.633 13
25 15.201 15
26 16.949 16
27 18.898 18
28 21.072 21 << 3 from 18 to 21
29 23.495 23 << 2 from 21 to 23
30 26.197 26
31 29.209 29
32 32.568 32
33 36.314 36
34 40.490 40
35 45.146 45
36 50.338 50
37 56.127 56
38 62.581 62
39 69.778 69
40 77.803 77
41 86.750 86
42 96.726 96
43 107.850 107
44 120.253 120
45 134.082 134
46 149.501 149
