about weaker engines

[Togga]

mostly the weaker engines are not really weaker , these are mostly only much slower, but not weaker in chess knowledge/ AI:

less nodes per second
less depth per time

or what are the differences between top engines and lower rated engines?

[hgm]

In can be anything. Some engines are weak because they have low nps, due to inefficient coding, use of an inferior programming language, or use of inefficient algorithms. Other engines are weak because they contain bugs. Most detrimental to strength is poor move ordering. Having poor or wrong knowledge in their evaluation can also be a reason. Some engines simply lack features (e.g. no hash table).

Micro-Max is weak because of a combination of effects: very poor strategic knowledge, (e.g. it cannot recognize passers, it does not have a mobility term in its evaluation), no move ordering to speak of, (causing a high effective branching factor), no piece list (slowing down its nps).