The little book is based on Richard Lee's method on determining necessary hardness, which is to make sure pressure doesn't exceed the yield of the alloy. This results in a harder number than most conventional formula's ask for, but Lee has some evidence of advantages for it. Read Lee's Modern Reloading, 2nd Edition for details.
My Lee tester reads higher than my LBT tester and higher than what I know about the alloy suggests it should read. I have not, however done systematic enough testing to eliminate all sources of error in the test, so my jury vote on it is still out. If the spring did not press the ball hard enough, that also would do it. The indentation diameter ratios, though, would hold, so once you measured a standard sample, like a bullet cast of pure lead (BNH 4.2) you could figure out the others.
Code:
BHN Indent diameter ratio to pure lead indentation diameter
4.2 1.0000
5 0.9225
6 0.8470
7 0.7873
8 0.7385
9 0.6979
10 0.6633
11 0.6335
12 0.6072
13 0.5840
14 0.5632
15 0.5445
16 0.5277
17 0.5122
18 0.4980
19 0.4850
20 0.4729
21 0.4617
22 0.4512
23 0.4413
24 0.4323
25 0.4236
26 0.4155
27 0.4079
28 0.4005
29 0.3937
30 0.3871
31 0.3809
32 0.3750
33 0.3694
34 0.3639
35 0.3588