container ruing CentOS7 on a Centos6 host?

I thought that the containers uses the same keel as the host... so I thought OS using different keel versions wouldn't work and I did a test to prove this:

I created a VM on virtualbox and installed CentOS 6 on it:

[root@42a752a105a3 /]# cat /etc/centos-release
CentOS Linux release 7.2.1511 (Core)
[root@localhost ~]# uname -r
2.6.32-573.el6.x86_64

Then I created a docker container with CentOS 7:

root@42a752a105a3 /]# uname -r
2.6.32-573.el6.x86_64
[root@42a752a105a3 /]# cat /etc/centos-release
CentOS Linux release 7.2.1511 (Core)

Obviously the keet of my container is the same one as the host (v2.6) but I don't understand why the CentOS 7 which was originally built around keel v3.10 workin on a v2.6 keel??

thanks