why there are differences between class files that manually created and created by eclipse

JavaCompilerTest.java

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.net.URI;
import java.util.Arrays;

import javax.tools.JavaCompiler;
import javax.tools.JavaCompiler.CompilationTask;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;


public class CompilerTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		CompilerTest ct = new CompilerTest();
		ct.testCompiler();
	}

	public void testCompiler(){
		//get source code from java file
		try {			
			//FileInputStream fis = new FileInputStream("source.java");
			FileReader fr = new FileReader("./src/Source.java");
			BufferedReader br = new BufferedReader(fr);
			StringBuffer sb = new StringBuffer();
			String line = br.readLine();
			while(line != null){
				sb.append(line);
				line = br.readLine();
			}
			line = sb.toString();
			
			//compiler
			JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
			//1) javaFileManager
			StandardJavaFileManager javaFileManager = compiler.getStandardFileManager(null, null, null);
			
			//2)javaFileObject
			URI sourceUri = URI.create("string:///Source"+JavaFileObject.Kind.SOURCE.extension);
			CompilerTest.StringJavaFileObject fileObject = new CompilerTest.StringJavaFileObject(sourceUri, JavaFileObject.Kind.SOURCE);
			fileObject.setContent(line);
			//put javaFileObject into list
			Iterable<? extends JavaFileObject> fileObjects = Arrays.asList(fileObject);
			
			//put them into task
			CompilationTask task = compiler.getTask(null, javaFileManager, null, null, null, fileObjects);
			boolean result = task.call();
			if (result) System.out.println("compile successfully!");
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
	static class StringJavaFileObject extends SimpleJavaFileObject{

		private String content = null;
		
		protected StringJavaFileObject(URI uri, Kind kind) {
			super(uri, kind);		
		}
		
		public void setContent(String content){
			this.content = content;
		}
		
		public CharSequence getCharContent(boolean
				ignoreEncodingErrors) throws IOException {
				return content;
		}
	}
}

source.class

// Compiled from Source.java (version 1.6 : 50.0, super bit)
public class Source {
  
  // Method descriptor #6 ()V
  // Stack: 1, Locals: 1
  public Source();
    0  aload_0 [this]
    1  invokespecial java.lang.Object() [8]
    4  return
      Line numbers:
        [pc: 0, line: 2]
      Local variable table:
        [pc: 0, pc: 5] local: this index: 0 type: Source
  
  // Method descriptor #15 ([Ljava/lang/String;)V
  // Stack: 2, Locals: 1
  public static void main(java.lang.String[] args);
    0  getstatic java.lang.System.out : java.io.PrintStream [16]
    3  ldc <String "Here's what you want."> [22]
    5  invokevirtual java.io.PrintStream.println(java.lang.String) : void [24]
    8  return
      Line numbers:
        [pc: 0, line: 5]
        [pc: 8, line: 6]
      Local variable table:
        [pc: 0, pc: 9] local: args index: 0 type: java.lang.String[]
}

source_manually.class

// Compiled from Source.java from StringJavaFileObject (version 1.6 : 50.0, super bit)
public class Source {
  
  // Method descriptor #8 ()V
  // Stack: 1, Locals: 1
  public Source();
    0  aload_0 [this]
    1  invokespecial java.lang.Object() [1]
    4  return
      Line numbers:
        [pc: 0, line: 1]
  
  // Method descriptor #12 ([Ljava/lang/String;)V
  // Stack: 2, Locals: 1
  public static void main(java.lang.String[] arg0);
    0  getstatic java.lang.System.out : java.io.PrintStream [2]
    3  ldc <String "Here's what you want."> [3]
    5  invokevirtual java.io.PrintStream.println(java.lang.String) : void [4]
    8  return
      Line numbers:
        [pc: 0, line: 1]
}

Sourece.java

public class Source {

	public static void main(String[] args) {
		System.out.println("Here's what you want.");
	}
}