Java HotSpot Virtual Machine

Hi everybody. I have two instances of oc4j on a cluster of two nodes. The configuration of parameters on opmn.xml is exactly the same for both instances.
Particulary I see an strange behavior on memory heap. I expect that if Xmx and Xms parameters are equals to 4096, then the heap size must be fixed on 4 Gb exactly, and there will be not resizing.
But monitoring the heaps with JVisualVM I can see that the heap size of the first instance is moving between 3.7 and 4 Gb, and the Heap size of the second node is moving around 3.2 Gb. The hosts have both 16 Gb of RAM, it's running on Windows 2003 Server R2 and there is no overload on the OS.
Possibly the behavior is normal and I just have a wrong understanding of what is expectable.
Here are some parameteres extracted from opmn.xml file:

-D64
-Dcom.sun.management.jmxremote
-Xmx4096M
-Xms4096M
-XX:+UseParallelGC
-verbose:gc
-XX:+PrintGCTimeStamps
-XX:+PrintGCDetails
-Xloggc:D:\oracle\product\10.1.3\soa\j2ee\OC4J_SOA\archivo_loggc.log
-Dcom.sun.management.jmxremote.port=8004
-Dcom.sun.management.jmxremote.authenticate=false
-Dcom.sun.management.jmxremote.ssl=false
-Djava.security.policy=D:\oracle\product\10.1.3\soa\j2ee\OC4J_SOA\config\java2.policy
-Djava.awt.headless=true
-Dhttp.webdir.enable=false
-Doc4j.userThreads=true
-Doracle.mdb.fastUndeploy=60
-Doc4j.formauth.redirect=true
-Djava.net.preferIPv4Stack=true
-Dorabpel.home=D:\oracle\product\10.1.3\soa\bpel
-Xbootclasspath/p:D:\oracle\product\10.1.3\soa\bpel\lib\orabpel-boot.jar
-Dstdstream.filesize=8
-Dstdstream.filenumber=30
-Dhttp.proxySet=false
-XX:MaxPermSize=768M
-XX:PermSize=768M
-Doraesb.home=D:\oracle\product\10.1.3\soa\integration\esb
-Dorabpel.process.lock.timeout=300
-Drmi.client.connection.timeout=100
-Doracle.dms.hunter.sleeptime=600000
-Doracle.dms.gatherers=300000,2:1800000:3
-Djavax.net.ssl.keyStore=D:\oracle\product\10.1.3\soa\jdk\jre\lib\security\cacerts
-Djavax.net.ssl.keyStorePassword=changeit
-DopmnPingInterval=90
-Xrs
-DHTTPClient.socket.staleCheck=true
-DHTTPClient.disableKeepAlives=true
-XX:+HeapDumpOnOutOfMemoryError