quality loss when using png as pattern

[Mirko Kunze]

Ubuntu 16.04, Inkscape 0.91

So basically I use a png image in two different versions. First by just opening it in Inkscape and storing it as svg, for the second by clicking "convert to pattern" and then storing as svg. The first looks like this in the file:

 <image
 width="1280"
 height="720"
 preserveAspectRatio="none"
 xlink:href="data:image/png;base64,iVBOR..."
 id="image10"
 x="0"
 y="0" />

and the second like this:

<defs
     id="defs6">
    <pattern
       patternUnits="userSpaceOnUse"
       width="1280"
       height="720"
       id="pattern4157">
      <image
         y="0"
         x="0"
         id="image10"
         xlink:href="data:image/png;base64,iVBOR..."
         preserveAspectRatio="none"
         height="720"
         width="1280" />
    </pattern>
  </defs>
  <sodipodi .../>
  <rect
     style="stroke:none;fill:url(#pattern4157)"
     width="1280"
     height="720"
     x="0"
     y="0"
     id="rect4160" />

Now, if I export both svg files as png using inkscape, the first one is identical with the source png, the second one becomes a tiny bit blurry, the color also seems ever so slightly darker and also the filesize shrinks from 1.7MB to 1.3MB. Why does this happen? Do patterned paths get resampled? How can I avoid this?