Why Hair and Fine Edges Are the Hardest Part to Remove Cleanly
Hair, fur, and similarly fine detail are the hardest part of an image to remove cleanly because individual strands are often only a pixel or two wide and semi-transparent at the edges, blending gradually with the background rather than having a clean, solid boundary — a segmentation mask has to make a binary or graduated decision at a scale finer than most models can reliably resolve, which is why hair edges often look slightly rough, missing fine strands, or retain a faint background halo compared to a solid-edged object like a box or a ball.
This is a genuine, well-known limitation of automatic background removal, not specific to any one tool.
Why solid objects segment more cleanly
An object with a solid, well-defined silhouette — furniture, packaging, most clothing — has a clear boundary the model can confidently classify pixel by pixel. Hair and fur don't offer that clean boundary; the transition between subject and background happens gradually over many nearly-transparent, wispy pixels, which is inherently harder to resolve with a single mask.
What actually helps get a cleaner result
A higher-resolution source photo, more even lighting, and a background with reasonable contrast against the hair color all improve the odds of a cleaner automatic result — for the cleanest possible hair edges in a professional context, manual refinement in an image editor is still sometimes used on top of an automatic first pass.