In a minor way in January and in a major way in June the game "re-ranks" the clubs in the game - that is, the reputation of the clubs changes depending on how well that team have done over the past 6 to 12 months.
Take Accrington Stanley. They would, at the beginning of the game be ranked as the worst League side (probably). If, however, they put a good run together, sign some decent players and are 5th come January the game will "re-rank" them - that is to say, they will now be seen by the game and by the opposition as a better team.
The upshot of this is that Accrington will probably get match-day odds in their favour in tight games and teams will play more defensively against them as they may expect to come up against a good side. Before, they may have attacked at will as they would see Accrington as three easy (or easier) points.
Throughout the game, however, match-day odds respond to form, and the AI also responds to form. If you're playing well, the opposition will set out as if they're going to have a tough game. If you're playing badly they'll fancy their chances.
Some swear (usually those with a grudge
) that re-ranking happens suddenly and indisctiminately in January and causes Manchester United to play like Manchester Working Man's Club. However, you'll usually find it's the weather and players getting jaded, which leads to a couple of defeats and poor morale and sometimes this can spin out of control. Adapting to weather conditions and keeping morale up should avoid any nasty complications. Besides, unless you go from nobodies to challenging for the title you shouldn't notice re-ranking even after the major one in June.
I have a feeling many will disagree with me, but I'm used to that at the moment.
Re-Ranking - What Is it? 
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