Different Behavior for Regex in XSD and XSLT/XQuery

I have this simple type in an XSD, which is my attempt to validate strings
that are subsets of the string representation of xs:time values:

  <xs:simpleType name="timestr">
    <xs:restriction base="xs:string">
      <xs:pattern
value="(\d|0\d|1\d|2[0123])(:([0-5]\d|60)(:([0-5]\d|60))?)?|24(:00(:00)?)?"/>
    </xs:restriction>
  </xs:simpleType>

And it correctly rejects "05:96" as invalid.

However, using the same regex with start/end anchors added in matches() it
does not reject "05:96".

This transform:

  <xsl:template match="/">
    <xsl:variable name="timestr" as="xs:string"
      select="'05:96'"
    />
    <xsl:variable name="is-valid" as="xs:boolean"
      select="matches($timestr,
'^(\d|0\d|1\d|2[0123])(:([0-5]\d|60)(:([0-5]\d|60))?)?|24(:00(:00)?)?$')"
    />
    <result>
      <is-valid><xsl:value-of select="$timestr"/>, <xsl:value-of
select="$is-valid"/></is-valid>
    </result>
  </xsl:template>o;?

Produces:

 <result><is-valid>05:96, true</is-valid></result>