--- cut here ---
<?xml version="1.0"?>

<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">

<fo:layout-master-set>
 <fo:simple-page-master master-name="page" page-width="297mm"
page-height="210mm" margin-left="1cm" margin-right="1cm" margin-top="1cm"
margin-bottom="1cm">
  <fo:region-body margin-bottom="1.5cm" margin-top="1cm"/>
 </fo:simple-page-master>
</fo:layout-master-set>

<fo:page-sequence master-reference="page">
 <fo:flow flow-name="xsl-region-body">
  <fo:table>
   <fo:table-column column-width="8cm"/>
   <fo:table-body border="black solid">
    <fo:table-row>
     <fo:table-cell>
      <fo:block>Don't care</fo:block>
     </fo:table-cell>
    </fo:table-row>
    <fo:table-row>
     <fo:table-cell border-bottom="black solid">
      <fo:block>Border bottom 1</fo:block>
     </fo:table-cell>
    </fo:table-row>
    <fo:table-row background-color="lightgray">
     <fo:table-cell border-bottom="black solid">
      <fo:block>Border bottom 2</fo:block>
     </fo:table-cell>
    </fo:table-row>
    <fo:table-row>
     <fo:table-cell>
      <fo:block>Don't care</fo:block>
     </fo:table-cell>
    </fo:table-row>
    <fo:table-row background-color="lightgray">
     <fo:table-cell border-top="black solid">
      <fo:block>Border top 1</fo:block>
     </fo:table-cell>
    </fo:table-row>
    <fo:table-row>
     <fo:table-cell border-top="black solid">
      <fo:block>Border top 2</fo:block>
     </fo:table-cell>
    </fo:table-row>
    <fo:table-row>
     <fo:table-cell>
      <fo:block>Don't care</fo:block>
     </fo:table-cell>
    </fo:table-row>
   </fo:table-body>
  </fo:table>
 </fo:flow>
</fo:page-sequence>
</fo:root>
--- cut here ---

When this FO is traformed into PDF, you have a bug with borders:
the bottom border of the cell "Border bottom 1" has disappeared,
hidden by the background of the cell "Border bottom 2".

But if you have a look at the top border of the cell "Border top 2", it did not
disappear
because of the background of the cell "Border top 1".

In fact, background in cells make any adjacent border-bottom to disappear.
I am afraid it is the same with border-left.

Note:
Add the 'background-color="lightgray"'
attribute to the very last "Dont'care" cell.
Then you see that the background hides a part of the border surrounding the table.

Note2: add the attribute 'background-color="lightgray"'
to the <table> tag. You will see that the background does not hide the borders
of the table,but is one pixel too large in the lower part.

PPS/ BTW, why does xpdf/acroread displays borders not always with the same size, but
the printed version is okay?