Varying Nutrition Values/Labels for the Exact Same Ingredient. How/Why?

It is something that's always bothered and confused me when reading nutrition labels and seeing different nutrition values for what's supposed to be the exact same thing.
I'll use Oat Bran as an example, since it was one of the things I bought today, that is like that between different brands.
4 different brands (2 well known brands here, and 2 store brands - from different super markets). All just plain simple oat bran. no fruits or mixes or other ingredients of any sort. At least not stated.
The sole thing stated is "Ingredients: 100% Oat Bran".
Yet the nutrition labels for 100g of it, all vary so much. Calories alone vary about 60-70 in difference between the lowest calorie label and the highest calorie label.
Protein varies from 6 grams up to 18 grams. Fiber varies from 4.5 grams on one, up past 14 grams on another.
(all for 100g amounts, just as that's easiest and quickest to compare, since most have different serving sizes).

What bothers and confuses me so much is why/how is it like that?
How can the exact same single thing have such a different in nutrition, just by different brand names on the packaging?
I can understand like tiny differences of a gram or two of one of the nutrients (and as such 4-8+ calorie differences too), as I assume growing conditions, soil and other stuff would make them all different by a little bit at most.
But I mean, the comparisons between the highs and lows of the different brands is just a bit hard for me to comprehend and seems ridiculous.
Are the labels wrong or lying?
Are they all somehow that different?
Which labels to believe if they are false?

Just looked up a picture of the packaging/label of the oat bran I bought today. The one online is different to the one in my cupboard, and not just by a single gram or something either.

Can anyone explain this to me? I know its pretty stupid. But in my mind it just bothers me.

And if nutrition truly does vary that much for the exact same things, then that completely changes which brands i'd purchase of at least half a dozen different things.
Cause a 3 gram difference in protein per serving between brands may not be much on its own. But if you buy 5-7 different ingredients all of brands that state a 3 gram higher protein count per serving compared to other brands of the exact same ingredient. That quickly adds up to a large amount of protein per day, that could decide whether you need to take a protein shake or not, or add in extra meat or protein sources to your next meal or not.
Same with calories. If 4 different ingredients had a 60 calorie difference from the highest calorie brand to the lowest calorie brand (according to their nutrition labels). Buying the lower calorie ones, and still using the same serving sizes and exact same meals, that's a 240 calorie saving on your day's limit. Which is pretty huge. Especially after a week of it.