The phrase for Isaiah 9:6 in the Septuagint (LXX) is "Mighty One".
Ezekiel 32:21 in the LXX is a little more complicated. The phrase doesn't exist, as it is implied by the use of the word giant (Nephilim - same as Genesis 6:4 "γίγαντες") whereas the Masoretic uses "the realm of the dead" (NIV) or "out of the midst of Hell" (KJV).
I'm assuming
@HTacianas is referring the references in Joel to a mighty army. Here the LXX uses innumerable or numerous.
So I think the answer to your question is that the original meaning focuses on the English equivalent "Mighty", where the various translators needed to imply context. (ie. "God" in MAS vs. "One" as in "The One" or "Only One" referring to the Messiah in the LXX) In the plural sense a strong army or strong people or giants that are mighty whether or not they be good or evil in Ezekiel and Joel.
Translators either focus on being as literal as possible, or they focus on what they think is the intended meaning based on their own experience or theology. There can be times when either method can theoretically be better, but you can't expect it to be perfect, hence we have hundreds of different translations.