I've had the tiniest nagging confusion about that...

From what I recall, it seemed pretty common to use the recipe on the bag of chocolate chips, yet somehow each family's cookies came out different.

My mom's instantiation of the bag recipe, for example, were pretty consistent across runs, yet not quite like anyone else's (that you're exposed to at friends' houses, school birthdays, bake sales, church potlucks, family reunions, cafes, ad photos, etc.)

Cookies are surprisingly sensitive to slightly different ingredients or practices. e.g. Using different brands of butter, different sizes of eggs, or storing the dough at different temperatures can have a large impact on the final product, even though the same recipe was followed.

I can speak to this - the main variations are in the kind of butter you use. Using salted, unsalted, or margarine result in similar yet different cookies. I personally use unsalted and feel like it creates the most “cookie” like experience. Flour brand and texture also makes a difference. You will get a very different result based on using the store brand vs (for example) King Arthur flour.

The tollhouse recipe is amazingly fragile. Slight variations in temperature can make the cookies "go flat" or end up with a thick skin. No surprise there is a variation of outcomes using older ovens that were very imprecise.

IIRC this would mostly be temperature offset + ease of oven temperature swings in response to introduction of thermal mass; plus humidity and altitude (= air pressure.)

(If you want to learn about reproducibility, look up what the factories making the packaged-snack version of your food tend to control for!)

Two main mistakes that people make: 1. "scoop and dump" approach to flour. Flour should be spooned into the measuring cup so its not packed in. 2. over-baking cookies due to cooking too long or oven that is too hot or not hot enough.

My brother set out to copy out mother's pie dough recipe, so had her do it in front of him. He recorded every measurement, each step of the creation process, but it didn't work. The result was a perfectly serviceable pie crust, but noticeably inferior to our mom's. Maybe the humidity in the kitchen, or subtle changes in rolling style... mom could do it every time and we couldn't copy it.

The truth is, we were trying to short cut past the effort involved of mastery by making hundreds of pies over decades...

Even if you use exactly the same ingredients, there is the temperature of the butter and how good a job you do of creaming it. Then there are factors like how do you mix the ingredients, and for how long. These all affect the final product. Then there is the oven, and if the temperature is accurate, if it does a good job holding temperature, etc. That's a lot of variables for something as simple as a tollhouse cookie!

Well I mean weight vs volume, actual oven temp, full fat vs skim, salted or unsalted. There's a lot of little variables even "following" a recipe.

Why are you using chocolate chips? Real bakers get the cacao pods from the grocery store, ferment the nibs inside for a month, then carefully formulate their own chocolate to pour into boutique chips of their own making.

As other comments point out, there is no added value in the pre-made ones, make your own chips! Oh, and the mixing bowl, why not take up glass-blowing? It's relatively easy to make your own bowls.