"All one length" is misleading. Everybody has hairs of every length between barely emerging from the scalp to the longest length. Old hairs shed out, new hairs grow in.
What "all one length" means is that you trim the bottom of the longest hairs to keep the hem as thick and even as possible. If you don't trim at all, you will get uneven ends because some hairs grow for a longer period of time than other hairs, also some hairs grow faster than other hairs.
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