It is indeed a matter of converting bits to bytes.
Manufacturers could put more room into a disk to get to an actual 1TB if they wanted to, but given that no one actually expects a 1TB drive to actually hold 1TB, they would have more costs producing the drive than people would know that their drive is actually holding slightly more data than other drives.
This 1,000 vs 1,024 conversion is a marketing trick used back in the day that became so standard, that it is now basically not a trick anymore, but it became the norm.
Going away from the norm only means that, as a manufacturer, you pay more to produce the drive.
Now, that said, I do believe there are some brands that actually produce drives close to the 1,024 conversion number, but there is no clear list or indication for which drives this is true or not.
When it comes for shopping for drives, look at the price and check reviews and make an informed decision based on that.