Keep in mind that the data isn't necessarily perfectly aligned on the track either. So maybe if the head is offset a little bit during the overwrite you might get an effect like this:
ORIGINAL
ORIGINAL
ORIGINAL
ORIGINAL
OVERWRITE
OVERWRITE
OVERWRITE
with a little strip of the original data still left intact off to the side a little bit.
Also as magnetization gets left alone for a longer time it is possible in an analogous way to 'soak in' wider / deeper physically into the media. So if you had something that was secret that had sat there for a year, and just last week you overwrite it, there will probably still be depth and width and bulk related traces of the original data in the physical magnetic statistics of the media.
Also keep in mind drives don't GIVE you physical access to the media anymore, not at least without very low level test/control software. Typically the drive decides how to map the physical surface into logical blocks.
So what happens if you write a full track of VERYSECRETSTUFF................VERYSECRETSTUFF......
and then you bump the drive and it gets a scratch / glitch in one of the blocks of that track? Typically then the drive will automatically relocate via remapping the 'bad sectors' to a new spare spot on the disc saved for this purpose. So now realize that the bit error that caused it to go bad might only be as little as one bit, or maybe even an intermittent problem. But the entire BLOCK (or multiple blocks) have been remapped. So now you CAN'T overwrite the VERYSECRETSTUFF......VERYSECRETSTUFF.... blocks because the drive doesn't even give them a logical block address anymore, it is just off limits in the "bad sector" list. But if someone with the right low level recovery software / hardware comes along and asks to read the bad sectors, chances are they'll be able to read back that data no problem.
Most data recovery software / hardware companies will generally milk you for whatever they can get, so almost anything costs over/near $10k, even the software tools. The hardware stuff can be up into the quarter million or more range. But people do buy / use this stuff. Heck there are even tools to take semiconductor IC chips apart atomic layer by atomic layer to reverse engineer the chip and the process technology. If they can do that, they can do a lot worse to a disc drive.
Also technology changes. What was state of the art 25 years ago, a 20MB MFM drive now would be just childs play to totally recover, it'd be less than 1/10,000 as precisely structured as the modern equipment commonly is. So what is a little difficult to do today, give it 5-10 years, and ......