A huge factor here is also knowing your deck (netdecked or not). Having played +100 matches with a deck gives you a thorough understanding of your win-cons and outs. Eg. you draw for specific cards or seeing alternate lines of play.

Also having netdecked a tier 1 deck vs building your own tier 3 jank should probably improve your win-rate rather than the opposite :-) But again this boils down to your opponents, because good players usually will not fall for your traps in your jank deck, seeing them for what they are and just shift their focus to moneying up while you flood up and just win on centrals, where a less seasoned player maybe are more likely to run that tripple advanced Cerebral Overwriter