why does our 5 yr old have trouble remembering some words, but not most (same number of syllables)?

Answer 1

Because of the way the human mind works.
Remembering is often a matter of associating the sound with something tangible. Your 5 year old may have made a solid connection for some words and a less firm one for others.
In any case, there is nothing to worry about. Soon there will be no way to stop his mouth!