The reason most e-liquid that is made in brick and mortar shops tastes different from PREMIUM liquid, is because it is NOT premium liquid.
First, let's be clear about what premium e-liquid is. There are two categories of e-liquid, as far as most are concerned. There is e-juice, and then there is premium e-juice. Regular e-juice (NON premium) is basically VG and/or PG, with or without nic, and 1-3 flavors. Often times they are simple recipes, such as Grape flavor, Butterscotch, Vanilla, Chocolate, Chocolate Pudding, Blueberry Cheesecake, etc. In order to create them, the seller basically buys the flavor such as "Grape" and adds it to the mix, and thats it. That is a regular e-liquid.
Now, a premium e-liquid takes experience, time, and trial and error. Now don't get me wrong....MOST brick and mortar shops will ATTEMPT to create premium e-liquids. The problem is.....they don't have the experience, they don't put in the proper time, and they certainly don't go through the extensive trial and error a company like Five Pawns, or Suicide Bunny, or Space Jam goes through. They might throw together 5-6 flavors, try to balance it a little until it tastes okay, and then slap a funny name on it, and sell it. Make no mistake....this is NOT a premium e-liquid.
A premium e-liquid isn't premium because it has more ingredients/flavors. EACH flavor must blend well, and the ENTIRE mix must be PERFECTLY balanced. When an e-liquid has a UNIQUE flavor profile, fused together from several different flavors, and balanced perfectly....it becomes a PREMIUM e-liquid.
Now, I hope that answers your question.
One example of a company that is local to me, that does what I mentioned above, is Vaperite (not to put anyone on blast, sorry guys). I order their Watermelon ALL THE TIME, because they make the best damn watermelon I've ever tried.....but when it comes to premium e-liquids, yuck. I'll stick to the companies that do it right
Again, not trying to put anyone on blast, my experience with vaperite isn't unique.....MOST brick and mortar shops don't compare to premium e-liquid vendors.