Shroppie Mileposts

The thirty-nine miles of the "Birmingham & Liverpool Junction Canal", now usually referred to as the Shropshire Union Main Line, between Autherley Junction and Nantwich are measured by a set of distinctive cast iron mileposts giving the distances to each terminal and, usefully for me, Norbury Junction! They are all to a standard design but there is a curiosity:
The first seven, between Nantwich and the top of Audlem locks, have black lettering on a white background. . .

Whereas the remainder through to Autherley Junction are all "white on black".

The changeover point happens to co-incide with the Cheshire / Shropshire county boundary, but I doubt that that is relevant, so if anyone does know why this happens, please let me know.