Why are the definitions of some characters not the same as the dictionary definitions of those characters?
This usually happens because there is a difference between the original meaning of characters and their common meaning today.
The meaning that a building block character had when the character writing system was being developed is often very relevant to the way that that building block is used in combination characters.
This is why these "radicals" (as they are technically called) are usually given "names" that correspond to their ancient meanings rather than to the modern ones. The meanings that Memrise introduces first for these characters is generally the radical one, because this in the one that is going to be most important in learning the largest number of other characters. Learning its common meaning later on is a relatively small task, as it is almost always derived from the ancient meaning.
When this happens there should always be another item in the database for the modern meaning, and there is a relationship between the items so that you will always be made of the alternative meanings.