Hello Christian, welcome to JavaRanch!
In the first instance: "t is an Object is: " + t
instanceof Object, the grouping is: ("t is an Object is: " + t)
instanceof Object. The object referred to by t is coerced to its String value and that value is joined onto the end of "t is an Object is: ", resulting in another String. That String is, of course, an instance of Object. So you get the result true printed.
In the second case you are evaluating in a different order: the object referred to by t is an instance of Object, so you get a boolean true. The boolean value true is coerced to the string "true" and that is appended onto "t is an Object is: ", resulting in "t is an Object is: true". That last String is what gets printed.
So no suprises here, really