The trend today is to have it a bit more centered. This one is “old school“. The reason for potting more centered is to help the roots stay even all around.
Here is a picture of the tree as it was on the cover of International Bonsai magazine in 1980:
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When comparing the 1980 image to the current one it appears that it is more centered than it used to be.
That image of a Zelkova has been imprinted on my brain for 40 years. For me, it’s the image of a “perfect Zelkova”, and mentally, whenever I see any broom style tree, I subconsciously compare it to this one. I have yet to see a better one.