There seems to be a growing misconception that the AI somehow "learns" your tactic over the course of a season or two, thus making your primary tactic obsolete. I have yet to see evidence for this, and by trying to find this evidence I have instead come across moderators on SI forums saying this is NOT the case.
Rather it's about how if you win games and/or trophies, your reputation will increase and your club will reach a higher stature. This will, in turn, lead to the other teams giving you more respect and often operating more defensively, becoming aware of your strong players and effectively closing them down or otherwise countering the threat they pose. Other influencing factors may be numerous and various, and include such things as player morale, tactics used vs tactics faced, playing home/away, high expectations, complacency etc.
To anyone who thinks I'm monumentally mistaken: Please correct me if I'm wrong about this, but if you do, please include some sources for your claims. People have so incredibly differing experiences with their games it's pointless to refer to anecdotes. I for one do not believe the AI in this game is so advanced that it learns your tactics (except for the isolated case of match to match), and even if it did, why would it take a year or even more? If a manager sticks to a primary tactic which he supplements with a secondary and tertiary tactic, most other managers will (in real life) figure this out alot faster than after a year. In other words, if the AI indeed IS that advanced, they somehow managed to apply it in a very stupid and unrealistic manner.
EDIT: Just to clarify, I'm thoroughly enjoying the game, sitting on the top of the league, and even in my 4th season the AI has yet to "learn" my primary tactic well enough to effectively counter it. I'm not trying to poke a stick at SI, but I am however seeking clarification regarding this myth.