In Agricola, Occupations cards are each marked with 1+, 3+ or 4+, indicating the number of players for which they are valid. For example, you cannot be dealt a 3+ or 4+ card in a two-player game. With a few exceptions (such as those that refer to the "Travelling players" space) the 3+/4+ cards would be playable, and potentially useful, in any game, but presumably they are restricted for purposes of balance.
Why do the Minor Improvements not follow the same system? There are a number of cards where their behaviour changes directly in response to number of players, especially in the I deck (e.g. Slaughter-house, Milking shed, Punner). Others don't directly refer to other players but still change in their utility, for example because of different availability of certain actions (e.g. Bake bread; see also here).
Intuitively, in designing the game I feel it would have made sense to have the same system for both Minor Improvements and Occupations: either decide that both should be available in all games, and accept that some will be more useful than others for a given number of players, or categorise them both in order to fine-tune the balance.
Is there any reason that they follow two different systems?