Four reasons, in the order they actually cost people fights.
1. Your troop ratio is wrong
Units attack the front row first. Too few Infantry means your Lancers and Marksmen are being hit on turn two instead of turn five. Power does not model this; the fight does.
2. Hero skills are not in the report
Century Games states plainly that hero skill bonuses are calculated only during battle and are not shown in battle reports. So a report where every listed number favoured you, and you still lost, is usually a skill you could not see.
3. You joined a rally and brought the wrong first skill
Century Games' own wording: a rally applies the initiator's 9 hero skills plus 4 primary skills from rally members. Read that carefully — those 4 are for the whole rally, not 4 from each person who joins. The game keeps the four highest-level primary skills across everybody in the rally, and recalculates them when a member who was supplying one leaves. So joining is a first-skill game, and it is a competitive one: A hero you invested heavily in for their third skill adds nothing when you are joining rather than leading — and a cheap hero with a strong first skill is worth nearly as much as an expensive one.
4. You attacked a garrison, not a player
In garrison defence the highest-bonus defender supplies the bonuses for everyone, and four more primary skills stack on top. The city you scouted an hour ago is not the city you are hitting now.
5. You brought duplicates and assumed they were wasted
The common advice is to diversify because "the same skill will not stack twice". Century Games was asked exactly this — if a rally member's 4 skills are the same as the captain's, do the effects stack? The published answer is yes, they stack. So an alliance that deliberately lines up the same first skill is not wasting anything. It is one of the few free damage multipliers in the game.
6. You judged field combat by arena results
Troop counters do not apply to Exploration or the Arena. That is the publisher's statement, not a theory. If your mental model of what beats what came from arena fights, it does not transfer to the field.
Sources: Combat FAQ — row order, rally 9+4, garrison defence · Hero skill bonuses are not shown in reports · Duplicate rally skills stack · Troop counters do not apply to Exploration or Arena · What actually raises Power
Point 1 is the only item here that is our inference rather than a quotation — it follows directly from the row order the same document describes.