hashcat and linux

Hi
I am new with hashcat.
I tested hashcat with mask on windows 8.1 and on Kali linux(virtual mashine)
both 64 bit

I started same command in both machines and I was wondering why windows with 8 cores takes longer
than virtual machine with 3.

little comparison and I noticed that in linux it only uses 6 character mask.

command I started in both is same(below)
hashcat -m 0 -a 3 md5_hash.txt ?a?a?a?a?a?a?a?a

mask is 8 characters long, but linux side it uses only 6, is there bug in linux version or am I missing something?

file contains 3 simple passwords that are hashed with md5