I think you can replace the word "hard" with "stiff" in a lot of cases.
The taper of the shaft can have a lot to do with this as can the makeup of the shaft. Laminated shafts tend to be stiff, or hard.
I was playing yesterday with my Tiger cue with the x-shaft. During the practice, I picked up a friends low end McDermott and hit a few balls. It felt like the shaft was made out of Balsa wood compared to the x-shaft.
Consequently, someone who is used to playing with that type of shaft like on the McDermott and picked up an x-shaft would think it hit like a brick.