Why a gif going through decode and encode will drop quality and size

[jiangyurong609]

I was trying to test decode and encode a gif and compare with original input file, I found the size drops and quality drops. is encode a lossy process? Or I miss something? below is my encode code

encoder.setRepeat(0);
      encoder.setDelay(decoder.delay);
      encoder.setDispose(decoder.dispose);
      encoder.setQuality(1);
      encoder.setSize(decoder.width, decoder.height);
      for (int i = 0; i < decoder.getFrameCount(); i ++) {
        encoder.addFrame(decoder.getFrame(i));
      }

      encoder.finish();
      assertEncodedImageIsEqualTo(gif);