Interconnection between I/O modules in the same fabric ?

Hello again,

trying to setup the I/O modules of my enclosure I noticed the following:

when I connected the fabric B1 switch (M6220) through an uplink to my existing network,

I was able to ping all my servers from my fabric B2 switch (M6220) which hasn't any uplink.

I am missing something?

If there is an interconnection between the I/O modules through the midplane?

and if that so, then why the manuals says that we have to stack or LAG the I/O modules in the same fabric?

Friendly,

Achilleas