As this quote
Quote:
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A few [of the Ents at the Entmoot] seemed more or less related to Treebeard, and reminded them [Merry and Pippin] of beech-trees or oaks. But there were other kinds. Some recalled the chestnut: brown-skinned Ents with large splayfingered hands, and short thick legs. Some recalled the ash: tall straight grey Ents with many-fingered hands and long legs; some the fir (the tallest Ents), and others the birch, the rowan, and the linden.
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shows they resembled the treeds, they reminded the Hobbits of the trees. Clearly there would be a difference between the Ents and the trees, but possibly this difference would diminish if the Ents grew treeish.
So it seems likely to me that it was due to their special assignment as shepherds and not as normal trees that they looked different, when they abandoned it they blended in with the others.
I know this sounds a bit strange, but as long as you have skin-changers capable of transforming into bears I don't see why Ents wouldn't be capable oftaking the shape of the trees they cared for.
Sounds quite like something Yavanna would think of.