SharpZipLib压缩出来为什么和WINRAR压缩出来大小不一样？

SharpZipLib压缩 和 WINRAR选择ZIP模式压缩 有什么不同？大小好象有点不一样。我主要压缩出来是改成JAR使用。JAR就是使用ZIP压缩方法，以前我都是用WINRAR选择ZIP模式压缩，我这次我需要批量处理一些东西，结果就出问题了。我用的SharpZipLib压缩代码如下：using System;
using System.IO;
using System.Windows.Forms;
using ICSharpCode.SharpZipLib.Checksums;
using ICSharpCode.SharpZipLib.Zip;
using ICSharpCode.SharpZipLib.GZip;namespace Nbgzc
{
    class ZipLib
    {
        Crc32 crc;
        ZipOutputStream s;
        string ZipDirectory;        public void ZipToFile(string _ZipDirectory, string _ZipFile)
        {
            if (!_ZipDirectory.EndsWith("\\")) _ZipDirectory += "\\";
            ZipDirectory = _ZipDirectory;
            string[] filenames = Directory.GetFiles(_ZipDirectory);
            string[] directorynames = Directory.GetDirectories(_ZipDirectory);            crc = new Crc32();
            s = new ZipOutputStream(File.Create(_ZipFile));            s.SetLevel(5); // 0 - store only to 9 - means best compression            CycleDirectories(_ZipDirectory);            s.Finish();
            s.Close();
        }        private void CycleDirectories(string _ZipDirectory)
        {
            string[] filenames = Directory.GetFiles(_ZipDirectory);
            string[] directorynames = Directory.GetDirectories(_ZipDirectory);            foreach (string file in filenames)
            {
                //MessageBox.Show(file);
                FileStream fs = File.OpenRead(file);                byte[] buffer = new byte[fs.Length];
                fs.Read(buffer, 0, buffer.Length);
                ZipEntry entry = new ZipEntry(file.Replace(ZipDirectory, ""));                entry.DateTime = DateTime.Now;                entry.Size = fs.Length;
                fs.Close();                crc.Reset();
                crc.Update(buffer);                entry.Crc = crc.Value;                s.PutNextEntry(entry);                s.Write(buffer, 0, buffer.Length);
            }            foreach (string directory in directorynames)
            {
                CycleDirectories(directory);
            }
        }
    }
}调用代码：
zl = new ZipLib();
zl.ZipToFile("Cache/" + BookID.ToString() + "/Jar/", "Cache/" + BookID.ToString() + "/" + BookID.ToString() + ".jar");
zl = null;