Why does ant compile all classes each run?

I'm more accustomed to make, so I'm confused why ant recompiles classes when the source hasn't been changed. I've read that there is a requirement to recompile in some cases where generics are used, but I'm not sure that this will be necessary for my project.

Also, in the javac task, I've set includeDestClasses="true"

Here's some of the targets I'm using

<target name="init">
        <mkdir dir="${build}"/>
        <mkdir dir="${dist}"/>
    </target>
    <target name="compile" depends="init,util,semantics" description=""/>
    <target name="util" depends="" description="">
        <javac destdir="${build}" classpath="project.class.path" debug="on" srcdir="${src}/util" includeDestClasses="true" source="1.5">
            <classpath refid="project.class.path"/>
        </javac>
    </target>