Not sure if this is a intended difference but reporting in case it is not.
Comparing javac man pages in jdk5.0u4 installation into Solaris and linux.
In Solaris javac man page, it says:
     -target version
           Generate class files that will work on VMs with the
           specified version. The default is to generate class
           files to be compatible with the 1.5 VM in the JDK.
           When the- -source 1.4 or lower option is used, the
           default target is 1.4. The versions supported by javac
           are:
...
           1.4 Generates class files that are compatible only
                 with 1.4 VMs.

           1.5 Generate class files that are compatible only
                 with JDK 1.5 VMs.

           5 Synonym for 1.5

But the same part in linux:
       -target version
              Generates class files that will work on VMs with the specified
              version. The default is to generate class files to be compati-
              ble with 1.2 VMs, with one exception. When the -source 1.4
              option is used, the default target is 1.4. The versions sup-
              ported by javac are:
...
              1.4 Generates class files that will run on VMs in JDK 1.4
                     and later, but will not run on 1.1, 1.2, or 1.3 VMs.

              1.5 Generate class files that are compatible only with JDK
                     1.5 VMs.

              5 Synonym for 1.5