Why is there absolutely no explanation of why certain US stamps have some words or values lined out?
The United States has issued very few stamps with words or
values lined out. These items are referred to as overprints. There
are several to check out in the catalog.
Some are called 'pre-cancels' and are intended to avoid having
to run them through a cancelling machine, but for the most part
this is un-necessary due to the automated machines currently used
for sorting.
Other than the pr-cancels and a few overprints (Kansas Nebraska
overprints, or Molly Pitcher) the US has never used it to re-value
a stamp.