The offset parameter is not an additional "start length", but how much of the dash pattern that will not be shown the first time. Although none of these are explicitly stated in the documentation, several (at least two, including me) have interpreted this as the latter.
Suggested improvement
Try to explain this better somewhere, ideally including a detailed example (now, all examples have a zero offset).
The offset operand can be thought of as the “phase” of the dash pattern relative to
the start of the path. It is interpreted as a distance into the dash pattern (measured
in user space units) at which to start the pattern. Before beginning to stroke a
path, the stroke operator cycles through the elements of array, adding up dis-
tances and alternating dashes and gaps as usual, but without generating any out-
put. When the accumulated distance reaches the value specified by offset, it begins
stroking from the starting point of the path, using the dash pattern from the point
that has been reached. Each subpath of a path is treated independently; the dash
pattern is restarted and the offset reapplied at the beginning of each subpath.
Activity
story645 commented on Jun 8, 2022
The Postscript Reference Book page 667 https://www.adobe.com/jp/print/postscript/pdfs/PLRM.pdf