A conforming cable can be up to 0.2mm larger than would fit into the Pibow. The rather weak excuse given by the designer is:
My Micro-USB cable doesn’t fit!
The Pibow supports a wide range of svelte, stylish micro USB cables, we recommend on [sic] of these. We may be able to slightly widen the aperture to include more cable. We don’t intend to support cables with large sleeves though, they just look naff 🙁
While I had the vernier calipers out, I measured all the USB connectors I could find. Not one was under 11 mm, way out of spec, and completely unlikely to fit in the Pibow.