You'd be surprised about how easily new winemakers with the jitters can get confused lol. These kit manufacturers design these kits so that they're almost fail-less. They 'balance' these kits, in terms of acidity and sugar, when diluted properly, and I'm rather sure they pay attention to the YAN levels as well; you may notice that most kits are using one of only a few types of yeast. EC-1118 being very common, K1V-1116 is another. Sometimes you'll find higher end reds with some thing a little more finicky, but if you're spending that much, you usually have done this a time or two already.
But these yeasts are almost fail proof themselves; they're strong fermenters, low H2S production and low nutrient demands.. it takes a lot to piss then off, comparatively. The actions of pouring these concentrates into a bucket, followed by the action of pouring water in behind them, then followed by the stirring of a few ingredients into the diluted kit - that's a good deal of oxygen, and pair that oxygen up with the balanced-out environment.. they design these kits for success (with the yeasts they supply)
"Set it, and forget it" comes to mind.
Which is all well and fine, because it works in that situation.
But you're also teaching yourself habits, like it's okay to snap the lid down on a new fermentation.
The day will come when, even if you're still making kits, you may do something as simple as change the yeast. Well if you pick a yeast that has a higher nutrient demand than what they designed the kit for, and your added nutrients are the same schedule as you used for the less finicky EC- 1118, I can practically promise you now that you'll crack that lid one day and get blown off your feet by H2S/rotten eggs smell.
(Note: if you change something as simple as the yeast, you may void the warranty; they can't guarantee success)
These kits are designed to be simple, and to work. They want sales, and positive feedback. They don't really care, design nor expect these to make world class wines. They want people to be satisfied enough, to get buzzed/drunk, and order another kit. Their instructions work with kits.
Parameters are a whole lot less perfect in the real world. If you get some under- or over-ripe grapes, the acidity-sugar balance will be off. Every year, in every vineyard, YAN levels are different no matter what any of us do. You can pull the same grapes off the same acre, for 5 years in a row, and every one of them will have different sugar-acidity-YAN numbers. You can time so that they're close, but something will be different every time.
Other fruits, say blackberries, elderberries, blueberries - they outnumber posts here of other fruits, because these fruits give people a harder time. It's a steep learning curve sometimes, depending on how hardly ingrained this 'kit mantra' is in the winemaker we're helping. They want to know the how's and the whys too, but they've made 12-15-20 kits and seen their process work every time. So it's Their process now, and they can't figure out why a kit manufacturer would teach them steps that wouldn't apply outside of those kits - I mean, you're still making wine right? Lol.. So the steps MUST be the same, it's only logical...
So basically, you can get away with it in a kit, but then we get the pleasure of trying to break your bad habits when you decide to step outside that small box.. it's a lot easier to teach you better wine making practices from the beginning than it is to fix a bad habit, especially if you're already interested in the how's and the whys..