lxml.html.document_fromstring is different from origin html document

[bringbladetodream]

I use lxml parse html to document.But lxml document is difference from browser.

import requests
import lxml
url='http://www.bjdch.gov.cn/n3952/n9279505/c10412743/content.html'
headers={'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.95 Safari/537.36 OPR/26.0.1656.60'}
html_string=requests.get(url,headers=headers,timeout=3).content.decode('utf-8','ignore')
doc = lxml.html.document_fromstring(html_string)
xpath_result=doc.xpath('//div[@class="header navBlock"]/h1/p')

xpath_result is None
And other xpath has result
doc.xpath('//div[@class="header navBlock"]/h1')
doc.xpath('//div[@class="header navBlock"]/p')

browser html like :
<div>
  <h1>
    <p>
    </p>
  </h1>
</div>
lxml html like:
<div>
  <h1>
  </h1>
  <p>
  </p>
</div>

May I ask what causes this.How to get ture document and use true xpath to parse html document.
Thanks.