sum() function giving strange result

Xml :-
<documents>
 <value>10.11</value>
 <value>20.22</value>
 <value>30.33</value>
 <value>40.44</value>
 <value>50.55</value>
 <value>10.11</value>
 <value>20.22</value>
</documents>

<xsl:variable name="SubTotals">
 <xsl:for-each  select="/documents/value">
   <value><xsl:value-of select="." /></value>
 </xsl:for-each>
</xsl:variable>

<xsl:template match="/">
   <xsl:apply-templates select="documents/value" />
</xsl:template>

The result :-
0
10.11
30.33
60.66
101.1
151.64999999999998
161.76