Here's my brother's first stab:
The piece in question may have a sense of development or "completeness" to its themes and structure that denote a finished arc, without actually taking us through one. Similarly, "Beginning" pieces may set up certain motifs without maturing them to satisfaction.
It's a subtle miss to the question, though; "finished arc" and "matured motifs" have no definite meaning in music theory; the phrases used ultimately describe impressions and not specific musical constructs. So it's basically saying nothing more than that 'Concluding' pieces feel concluded.