Open two tarot books and you will eventually find them contradicting each other about Strength. In one it is card VIII; in the other it is XI. The same swap has happened to Justice in the opposite direction. Neither book is wrong, and neither will usually tell you that the other exists.
What actually happened
The older Continental pattern — the one that survives in Marseille decks — numbers Justice as VIII and Strength as XI. That is how the trumps were numbered for centuries, and if you buy a Marseille deck today that is what you will get.
In the late nineteenth century the Hermetic Order of the Golden Dawn built a system attaching each trump to a sign of the zodiac, in order. Run the twelve signs against the trumps in sequence and the eighth position lands on Leo. Leo's animal is the lion, and there is exactly one card in the deck with a lion on it. So Strength moved to VIII, Justice moved to XI, and the sequence lined up with the sky.
Arthur Waite was a member of the order. When he and Pamela Colman Smith produced their deck in 1910 they followed the Golden Dawn numbering, and because that deck became the most reproduced tarot in the English-speaking world, most English-language tarot writing inherited the swap without comment.
Why it is not a trivial detail
If the numbering were only a label it would not matter. But the Golden Dawn moved the cards because of an attribution, and the attribution carries interpretive weight. Under that system Strength is a Leo card: solar, warm, concerned with instinct rather than force. Justice is a Libra card: relational, concerned with proportion between people. Those readings come from the signs, not from the pictures.
Work with a Marseille deck and you inherit neither. La Force and La Justice are read from their imagery and their position in the sequence, and the zodiacal layer simply is not there. That is a coherent way to read. It is just a different one.
How to tell which system a book is using
Check the number on Strength. If it is VIII, the book is Golden Dawn descended — and you can expect it to attribute signs and planets to the other trumps as well: Aries to The Emperor, Taurus to The Hierophant, Scorpio to Death. If Strength is XI, you are in the older tradition and those attributions will be absent or borrowed from somewhere else.
Neither is more authentic. The Marseille numbering is older; the Golden Dawn system is more internally consistent, because it was designed to be. What causes trouble is reading a Marseille deck with an RWS book, or the reverse, and concluding that one of them is defective.
The rest of the disagreement
Once you know to look for it, the same fault line runs through the deck. Thoth decks rename the trumps — Strength becomes Lust, Justice becomes Adjustment, Judgement becomes The Aeon — and shuffle the court ranks so that its Knight corresponds to the RWS King. The Marseille pips are unillustrated, so the scenic readings most people know for cards like the Three of Swords are a Smith invention rather than a tradition.
Our card pages state which tradition each name and attribution belongs to, and the astrology section works through the Golden Dawn attributions one at a time, including why each pairing was made. If you want the short version: know which system you are in, and stop expecting the other one to agree.