GDG- Unknown known soldier

[John Gross]

A footnote on page 302 of "The Last Full Measure" by Richard Moe says

"According to National Park Service records at Gettysburg, Isaac Taylor's
remains lie in grave 12, row B, of the Minnesota plot. The headstone on the
grave is marked 'Unknown.'"

Why is the graved marked Unknown? Are there other unknown known graves at
the cemetery?

Thank you,

John Gross
confederate at att.net