That is true. Its why it is so easy for government and police agencies to "recover" hard drive information.
think of your hd as a notebook. You always have a certain amount of space on your hard drive once its formatted. The computer will write in this space, and what you delete is not "removed" from the notebooks pages and replaced with equivalent space. It would be more accurate to think of as the computer marking that space to be written over. In effect, you can't access it anymore, but it is still there until you write something onto the harddrive and the computer writes OVER the old data.
To think of the notebook situation, it would be like this. Say you write something on page one, and decide later you don't want it, when youre already on page 50. It's quicker for the computer to access the initial parts of the drive, so when you want to write page 51, it would be the equivalent of going back to page one (where you've marked it for deletion) and whiting out the page or writing over the old information with the new.
This is why true data erasers write OVER the old spaces where the data was, several times, to ensure that no old/original data still exists, but is instead filled with random junk. Most hackers, pedophiles, or other coputer criminals dont have time or the means to do this, but the point is: there is no way of truly erasing a hard drive or volume--short of destroying it or writing over it with new data.