First: For tomcat, you should configure CATALINA_OPTS, not JAVA_OPTS, but this is unrelated to the question, just general best practice.
In memory management it might happen that the JVM allocates some memory but does not provide every single byte to the application as heap space. As a Maximum this JVM is configured to have 1024M and the memory meter shows that the JVM can provide up to 954466304 bytes to the running application. It has not yet allocated all that memory from the OS (rather only 869400576 bytes) and without asking the OS for more memory, this is what is available to the application. After the last garbage collection, when some of the memory has been freed up, Java, Tomcat and Liferay are now using 761741872 bytes of this available memory.
In production environments I recommend to set -Xms and -Xmx to the same value: If, sooner or later, java couldn't allocate up to -Xmx memory from the operating system, you'll probably like to get this message right when you start your server than sunday night at 3 o'clock when the server fails unexpectedly with an out of memory condition.
You might want to read about general java memory handling, which might even be specific to the brand and version of the JVM you're using - the configuration above is (at least) valid for the Oracle implementation of Java7. And your production server will most likely have more finetuned values for the garbage collector and other aspects of the JVM, not just the max. memory.
