I know in the case of TM Century, they did some vinyl transfers and a TON of homemade edits to get the "hit" versions. BEWARE! I've spent countless hours going through the homemade edits on TM's library (even the newer, revised versions of the discs), and there are countless examples of careless mistakes. I'd be willing to bet that more than 75% of the homemade edits on TM's library are incorrect.
Does Michael Jackson's "Human Nature" run 3:45 and have an extra synth at 1:58? The hit 45 version of this one is edited and has a synth overdub that the LP doesn't have. Does "Thriller" have the rapless ending like on the 45? Does "Another Part Of Me" have the 45 remix or is it the same mix as the LP?
Does Janet Jackson's "Nasty" run 3:44 like the 45? Is Hall & Oates "I Can't Go For That" the hit 45 remix or is it the LP edit? Does Marvin Gaye's "Sexual Healing" run 4:07 like the 45, or is it the shorter 3:59 LP version?
Be careful in assuming you have the "hit" versions of all these songs until you've investigated their homemade edits. Many of these companies did a rush job of putting them together, and didn't take extra care to get it exactly right. I've even found instances on my TM discs where the song appears to be correct (i.e. it has the correct run time and correct sections edited out), but when doing a careful comparison to the actual vinyl 45, I found that the edit point was off by a measure or two. For example, "Lucky Star" on their disc is the same length as the 45, but the intro is not edited the same way as the 45. Someone put the edit point in the wrong spot.