--sequential-read not sequential???

I am confused and could do with some help.

I want to check (--test) my main DAR backup archives to make sure they
have not been corrupted. Unfortunately, the NAS that stores the archives
can't run dar, and my desktop doesn't have enough diskspace to store all
the slices at once. So, I have created a simple script to use -E to copy
each slice over as it is needed, and delete it when the next one is
requested.

As the copying takes quite a long time, I decided to use
--sequential-read so that the slices only get used once each. So, my
command looks like:

dar -t "/var/tmp/$archive" \
	-E 'rm "%p/%b."*".%e"' \
	-E "echo Fetching $dir/%b.%n.%e.gpg..." \
	-E "gpg -d -q -o '%p/%b.%n.%e' --batch  '$dir/%b.%n.%e.gpg'" \
	--sequential-read

This seems to work. I get the following output at the end, which I
presume means success (a simple yes/no summary of the --test would be
nice to add, by the way :-) ):

 --------------------------------------------
 5661770 item(s) treated
 0 item(s) with error
 0 item(s) ignored (excluded by filters)
 --------------------------------------------
 Total number of items considered: 5661770
 --------------------------------------------

HOWEVER, looking at the output of all the "Fetching" messages, I notice
that after reading all 117 slices in order, dar fetched slices
1,8,9,10,11,13 and 117 again!

Shouldn't --sequential-read prevent that? Or am I misunderstanding how
it works?

Graham