Target used by x other policies

Why am I getting this error.

If I go into a policy and create a new target to send it to 1 PC using 'exclude computers not in - Computer List - hostname"

I save it - 10 minutes later I go into it -

I then go to add another Pc into this target (computer list)  and I get message " the target is used by xx other polcies"

How can it be if I just created it!