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Inexplicable (adjective)
Impossible to explain; not easily accounted for.
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Unexplainable (adjective)
That doesn’t have a known or readily available explaination.
Wiktionary
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Inexplicable (adjective)
unable to be explained or accounted for
“for some inexplicable reason her mind went completely blank”
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Unexplainable (adjective)
unable to be explained or accounted for
“he was subject to unexplainable rages”
Oxford Dictionary