I had this in another comment, but there's a reasonable explanation for TK vs TC:

The letters TC appear next to each other in common words such as "watch" and "stretch." So if you were to use the ctrl+F function to search for those letters together, you could come up with many words that contain them, whereas TK is much, much rarer. The only example I can think of would be the word "pocketknife," which is nowhere near as common as words containing -TC (switch, catch, kitchen, itchy, postcolonial, sitcom, glitch, farfetched, catcall, etc). So if you search for TK in a document, you'll only find your notes.