a place I could buy King Arthur? I have been hearing a lot about how Japanese flour just does not have the gluten content that American flour has. Have you confirmed that the users surely chose strong bread flours? I presume not.
Some or all of them might have used a weaker floor which was not good for making breads.
Just as you were clueless before getting information from the above two posters, they did not know from the start about Japan-manufactured flours.
A Japan-manufactured flour which is categorized under 強力粉 [ kyouriki-ko ] has a protein content of about 11 -13 percent (or maybe more).
It is advertised in the following website that King Arthur Unbleached Bread Flour has a protein content of 12.7 percent, a full point higher than other U.S.A. brands.
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http://www.kingarthurflour.com/So, you should admit that the gap is not a problem if suitable flours are chosen.
(You say "gluten content" but you cannot weigh the quantity of gluten before adding water to the flour.)
The Japanese word for "bread" is "pan," which is considered to come from Portuguese, but maybe you had better avoid this word in asking at a shop; clerks might misunderstand that you are searching for bread crumbs, called "pan-ko" in Japanese.
Keywords are " 小麦粉 [ komugi-ko ]" {flour}, " 強力粉 [ kyouriki-ko ]" {strong floor}, "making breads" and "over 1 kg."
Frankly speaking, you seem to be thinking as if the same flour of the same brand always brought gluten of about the same quantity and resulted in bread of about the same quality.
If you believe so, you have been just lucky in making breads, I suppose.
Enjoy making breads in Japan, through trials and errors!