Why Acrobat X tool "Convert Colors" adds compression artifacts?

I'm trying to convert some CMYK PDFs in sRGB, but when I use the "Convert Colors" tool in Acrobat X Pro I got also a lot of compression artifacts.

I've made a very simple PDF to show the problem.

Step 1:

Step 2:

Step 3:

Is it my fault (in some way) or is it a bug?

Thanks!