db:attribute and eq unexpected behaviour

form13FFileNumber="028-17473"
form13FFileNumber="028-16573"

let $data := db:open('13F')//data[@form13FFileNumber eq $form13FFileNumber]

xquery version "3.0";
declare namespace db="http://basex.org/modules/db";
declare namespace file="http://expath.org/ns/file";
declare variable $form13FFileNumber as xs:string external;

(:let $data := db:open('13F')//data[@form13FFileNumber eq
$form13FFileNumber] :)

let $fields :=

<form13FFile>
{
for $c in $data return $c

}
</form13FFile>

return file:write(concat('../OUTPUT/form13Restatment3rd_' ,
$form13FFileNumber , '.xml'), $fields)

<?xml version="1.0" encoding="UTF-8"?>
<commands>
    <set
option='attrinclude'>xml:id,filingDate,type,cik,sequence,form13FFileNumber,periodOfReport</set>
    <create-db name='13F'/>
    <info-db/>
</commands>

The xml element that has all the indexes
 <data xml:id="0001534270-16-000006" filingDate="2016-06-08" type="13F-HR"
cik="1534270" sequence="000006" form13FFileNumber="028-17473"
periodOfReport="03-31-2016">

let $docs := collection($path)
for $doc in $docs
return
let $path :=
concat($doc/data/@periodOfReport,'/',$doc/data/@form13FFileNumber,'/',$doc/data/@xml:id,'.xml')
return db:add('13F', $doc, $path)