Table borders disappear in default or low zoom ratio

I used css to draw table borders(source is below), and PD4ML generated PDF file was OK, and paper printed via printer was OK, too.
But when I used Adobe Acrobet Reader 9 to view pdf file genereated by PD4ML, in default zoom ratio or low ratio, some borders “disappear” !? (see attached file)
If I changed zoom ratio to greater such as 150%, those disappearances displayed again…and I noticed those borders are slimmer than others.

BTW, there’s something I noticed,
if I remove padding or background style of th/td, everything is OK
but when I use padding and background style, some borders will disappear in low zoom ratio.

PD4ML ver: 360fx1 registered and 361b1 trial

Html source:

<html>
<head>
<meta http-equiv=Content-Type content="text/html; charset=utf-8">
<style type="text/css">
.tb1a{border:1px #000 solid; border-collapse:collapse;}
.tb1a th,.tb1a td{padding:3px 3px; border:1px #000 solid;}
.tb1a th{background:#ddd;}
</style>
</head>
<body>
<table width="100%" border="0" cellpadding="0" cellspacing="0" class="tb1a">
<tr>
<th>111</th>
<th>222</th>
<th>333</th>
<th>444</th>
<th>555</th>
<th>666</th>
<th>777</th>
<th>888</th>
</tr>
<tr>
<td>aaa</td>
<td>bbb</td>
<td>ccc</td>
<td>ddd</td>
<td>eee</td>
<td>fff</td>
<td>ggg</td>
<td>hhh</td>
</tr>
</table>
</body>