why [Saved][D] instead of [Delta][D]?

In another use of dar on a non-encrypted filesystem, I am using options
--delta sig and --delta-min-sig-size 1M.  I have a full and a
differential with that full as --ref, both with the same --delta..
options.  However, when I list the differential's contents, all [D]
files (those with computed deltas) in it are [Saved][D] instead of
[Delta][D]. Why might that happen?