In a nutshell: Sparse file.

After you read that, try this script to check whether a file is sparse or is not sparse:

#!/usr/bin/perl

use strict;
use warnings;

my $file = shift @ARGV;
my(@status) = stat $file;
print "\n";
if ( $status[7] > ($status[12] * 512) ) {
    print "$file is sparse\n\n";
}
else {
    print "$file does not appear to be sparse\n";
}
system("du -hS --apparent-size $file");
system("du -hS $file");
print "\n";
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The apparent-size file will usually be lt than the actual-size. If the apparent-size is gt the actual-size, that might be a sign that something's wrong.