Ledger line sizes - a question from simple curiosity

[Paul Hodges]

When specifying the thickness of ledger lines in LilyPond, one specifies not one but two multipliers, so that the result is proportionate to both staff line thickness and staff spacing.  E.g. (from the manual):

    \override StaffSymbol.ledger-line-thickness = #'(0.5 . 0.4)

For my own use I've simply used the first number to specify what I want and made the second 0.0 which seems to do fine.

I'm just wondering why a single proportion was considered insufficient...

Paul