...So my question is do they have something we don't know about? ...
It is simply experience. Until DIY is like making cake from a box and less like making carrot cake from scratch, experience means quite a bit. You are well on your way.
Justboringsarah has some good hints. There are other things you will discover as you go, the one that was the hardest for me was getting a clue as to where to start with flavoring ratio that would get it close for me. I have gotten to the point that I can usually get close on the first try based on the flavoring's smell alone.
It isn't the simplest thing in the world, but anyone can do it if they keep notes and keep trying. Realizing that each time you make a batch that doesn't work for you, you have been successful. Each batch that doesn't work is discovering another way to NOT do it again. I have dumped many pints down the drain, 3-6ml at a time, and filled two notebooks. But it was all fun. I enjoyed myself even when making the worst sweat-socks-dipped-in-eel-slime tasting juice known to man.
But, never give up on a mix just because it doesn't taste right right-away. When I can taste something close to what I wanted, I set it back for a week. I come back to it and see if it has improved, gotten worse, or stayed the same. Then I see if I can fix it. Maybe it needs some more flavoring? Maybe it needs some MTS? Maybe a completely different flavor? Maybe some mint, or menthol, or PGA? Maybe it needs to be dumped and notes updated to Do-Not-Try-Again?
The main idea I offer, is to keep trying.