It seems that the underlying dictionaries used by the three functions in the title are different. My ultimate question is "Why?"
As an example, I stumbled upon this set of results:
DictionaryLookup["gage"] returns {gage} (it recognizes the word);
WordData["gage"] returns {{"gage", "Noun", "MeasuringInstrument"}, {"gage", "Noun", "Marihuana"}, {"gage", "Verb"}}, though
DictionaryWordQ["gage"] returns False.
It seems that the last function is referencing a different dictionary than the first two (for what it's worth, DictionaryLookup works immediately; WordData loads a database before responding.)
This problem is referenced in this question, but the point of that question is different.
My question: why are there different (English) dictionaries?
FalsewithDictionaryQ. Most of them are proper nouns and a lot of inflected forms but also things like "decompiler" or "voicemail". Hmm, hard to tell if this was made on purpose. – halirutan Jun 29 '18 at 04:57