At first sight, Ollivander scowled at the wand, and even before he touched it, he commented, "This is not the wand I originally sold you." Out of reflex, he looked to the shelf where he knew that wand was kept, tucked along with all the others of its classification, but then sadly remembered he wasn't in Diagon Alley and all his shelves were bare.
Holding the wand and turning it nimbly in his fingers to inspect it, he saw that there was nothing intrinsically wrong with it; he held it up and with a swish, "Duro!" he turned his empty teacup on the table into stone. "Hmmmm... it's in perfect order. It must be ill suited for you. What ever became of your first wand? 11 inches, Willow, Phoenix feather core? And how did you receive this one? I most certainly wouldn't have sold it to you had the wand not responded to you at the time of purchase."
His guess that Dennis might've gotten it from some classmate who he'd challenged to a duel, possibly at Hogwarts, and that he'd sold this wand to some witch or wizard some time in his future, but not so distant, since Dennis wasn't that much older than from the time he first came into his shop.