Robert Helenius is the perfect example of this. His KO record (19 KOs in 30 wins and 33 fights) doesn't look impressive at all. On paper. But in reality he hits very, very hard and has proven it many times. Helenius has fight-ending power in both fists and is one of the hardest punchers of the last decade in the HW division. Name some other guys whose power in reality is much better than it looks on paper (KO% in their records)
Edner Cherry is a pretty good example. Cherry always had the heavy hands but did not have the delivery system.
Duran's rise through the divisions did a number on his KO% The era Jimmy Wilde fought in (when you had to nearly murder someone for the referee to step in) didn't help his knockout rate. Lennox punched like an absolute mule and 32 KOs in 41 wins is still very respectable but could have been even higher if he hadn't taken the foot off the gas (or never fully put it down) in a few fights. Oh and fighting a human bowling ball like David Tua will never help your KO% either.
Chris Eubank. I think because he was more of a counter puncher and because he didn't always push for the KO, especially after the Watson fight, combined with not always setting a high pace saw his KO percentage not reflect his true punch power. He dropped iron chinned Collins for example and turned the fight around vs Watson with one punch, also hurt Cruiserweight Thompson.
Dick Tiger comes to mind. He had a low ko percentage but was regarded by his opponents as a heavy handed dude. No doubt he could crack.
James Toney is another guy. He had a 51% ko percentage but he definitely could hit hard enough to make heavyweights uncomfortable even though he was a naturally smaller guy at 5'9.