This means x1.fx=9 is exactly doing x1.lo=9 and x1.up=9 in one statement.... GAMS sets the lower and upper bound to that value.
Additionaly, this kind of statements (setting variable attributes) are executed in order. So when you do x1.lo=8 afterwards, you are "overwriting" the previous set of the lower bound.
Best regards!
Claudio