The favourable reading of hexagram 15 is not the mild thing its English name suggests, and the fifth and sixth lines are where that becomes obvious.
The fifth line speaks of not boasting of wealth and yet being able to attack, and the sixth of modesty that makes itself felt, with armies set in motion to discipline one's own city and country. Wilhelm keeps both. These are not counsels of self-effacement. They describe a person whose position is strong enough that they do not need to advertise it, and who can act decisively without having first made claims.
That is the hexagram's real content and it is much more interesting than humility in the devotional sense. The advice is about not spending capital on display: keep the resources unannounced, and they remain available.
In reading practice the figure is generally taken as confirmation that a low profile is correct, that the querent's position is better than it looks, and that claiming credit now would cost more than it returns. It is also one of the most reliably encouraging results a consultation can produce, for the structural reason given already, there is no line in it that goes wrong.
Translations diverge here mainly on register. Wilhelm's superior person, Legge's man of virtue and Rutt's flatter rendering describe the same behaviour with very different amounts of moral scaffolding, and a reader comparing them will learn more about translation than about the hexagram. That is not wasted effort: knowing how much of a reading comes from the translator is most of what distinguishes careful use of this book from credulous use of it.