A page ranks for a hard term but misses an easier related one because ranking is decided per query, not in order of difficulty. Your page matches the exact intent behind the hard term well enough to win it, while the “easier” term carries a different intent or is simply better served by another page, yours or a competitor’s. Relevance is query-specific, so missing a term that looks simpler is an intent or coverage mismatch, not a paradox to be solved by difficulty math.
The trap is assuming that beating a hard keyword should hand you every easier one in the cluster. Google evaluates each query against the page that best answers that specific search. The hard term and the related term can sit close in your mind and still represent two different jobs: one might be informational where the other is transactional, one might expect a guide where the other expects a tool or a list. Your page was built, knowingly or not, to satisfy the first job. For the second, it reads as a near miss, and a page that fits that intent more squarely wins instead.
There is a second common cause: the easier term may already be owned by another of your own pages, and Google has decided that page is the better representative of your site for that query. In that case you are not absent from the term, you are competing with yourself, and the engine routed the query to the page it judged a closer match. This is ordinary observed behavior, worth checking in your own search data rather than reading as a glitch.
To act on it, pull the live results for the easier term and read what ranks. Decide whether the intent really matches your page, or whether it calls for a different format, a different angle, or a dedicated page of its own. If the right owner is another page on your site, strengthen and link to that one rather than forcing the cluttered page to chase a query it was never shaped for. Match the page to the intent, and the “easier” term stops looking like a contradiction.