Testing compression speeds using the flags from your benchmark seems about the same on my machine. Maybe your version was compiled differently? Or maybe a strong machine dependency?
Code:
~$ time 7za a -t7z -mx6 linux-3.13-rc7.tar.7z linux-3.13-rc7.tar
7-Zip (A) [64] 9.20 Copyright (c) 1999-2010 Igor Pavlov 2010-11-18
p7zip Version 9.20 (locale=en_US.UTF-8,Utf16=on,HugeFiles=on,8 CPUs)
Scanning
Creating archive linux-3.13-rc7.tar.7z
Compressing linux-3.13-rc7.tar
Everything is Ok
real 3m9.577s
user 5m21.212s
sys 0m1.864s
~$ time xz -z -6 -k linux-3.13-rc7.tar -v
linux-3.13-rc7.tar (1/1)
100 % 76.0 MiB / 524.3 MiB = 0.145 1.7 MiB/s 5:13
real 5m13.160s
user 5m12.584s
sys 0m0.572s