out of curiosity

I'm not new to TTC or the whole TWW thing but I've always been curious to know something...

If someone is in their TWW & their egg isn't meant to implant until 9dpo, then how can they start symptom spotting and notice changes before implantation takes place? Now, I've done the whole symptom spotting thing and I've noticed stuff at 3po but I know that's way too soon for anything to happen. Is it a rise in hormones? There's a rise in hormones before AF, right? So how can you spot the difference between the two?

We're TTC next month, & I want to know this time around so I won't start looking for signs too early. Just never understood how or why symptom spotting is good. Even when I did it, I regreated it after the TWW was over because I spent so much time looking for signs that were just AF.