The squared norm of w is exact rational arithmetic. Turning that squared norm into a numeric width uses a square root, so the width is rendered as a named radical symbol rather than a decimal.
highlighted = computed this step
The exact squared norm
The squared norm is w·w=1/2. This value is exact rational arithmetic from w=(1/2,1/2), b=-1.
w⋅w=1/2
Why the width is named
The margin width is 2/√(1/2). The square-root step is the named boundary, so no decimal width is pinned.
width=2/1/2
Summary
The geometry values are exact; the numeric width crosses the square-root boundary. The renderer keeps it as a symbol.