Did a little research. I hate research. Ledger lines are lines
in a ledger, which originally meant a book which was supposed to
remain at a certain location and not be moved about.
Leger lines are light lines, not heavy lines. LilyPond makes
leger lines heavier than staff lines to compensate for their
being shorter. That increases general legibility at the cost
of making them harder to distinguish from staff lines in some
situations. You can't have everything, and the most important
thing is to have them a different thickness whether heavier or
lighter. IMO they are optimum.
Ledger and leger are different words, with different meanings and
different derivations from different languages. The confusion of
leger with ledger is not merely a spelling error. Regards, daveA