spec/plugman/projects/android/cordova/lib/prepare.js
function default_versionCode(version) {
    var nums = version.split('-')[0].split('.');
    var versionCode = 0;
    if (+nums[0]) {
        versionCode += +nums[0] * 10000;
    }
    if (+nums[1]) {
        versionCode += +nums[1] * 100;
    }
    if (+nums[2]) {
        versionCode += +nums[2];
    }

    events.emit('verbose', 'android-versionCode not found in config.xml. Generating a code based on version in config.xml (' + version + '): ' + versionCode);
    return versionCode;
}
function default_versionCode(version) {
    var nums = version.split('-')[0].split('.');
    var versionCode = 0;
    if (+nums[0]) {
        versionCode += +nums[0] * 10000;
    }
    if (+nums[1]) {
        versionCode += +nums[1] * 100;
    }
    if (+nums[2]) {
        versionCode += +nums[2];
    }

    events.emit('verbose', 'android-versionCode not found in config.xml. Generating a code based on version in config.xml (' + version + '): ' + versionCode);
    return versionCode;
}
Consturct the default value for versionCode as PATCH + MINOR * 100 + MAJOR * 10000 see http://developer.android.com/tools/publishing/versioning.html