Query vs. Query Futures - Response Time Inaccurate?

I have Python code that fires 10 GET requests and measures the response time:

from datetime import datetime
from requests_futures.sessions import FuturesSession
import requests

class CustomSession(FuturesSession):

    def __init__(self, *args, **kwargs):
        super(CustomSession, self).__init__(*args, **kwargs)
        self.timing = {}
        self.timing = {}

    def request(self, method, url, *args, **kwargs):
        background_callback = kwargs.pop('background_callback', None)
        test_id = kwargs.pop('test_id', None)

        # start counting
        self.timing[test_id] = {}
        self.timing[test_id]['cS'] = datetime.now()

        def time_it(sess, resp):
            # here if you want to time the server stuff only
            self.timing[test_id]['cE'] = datetime.now()
            if background_callback:
                background_callback(sess, resp)
            # here if you want to include any time in the callback

        return super(CustomSession, self).request(method, url, *args,
                                                  background_callback=time_it,
                                                  **kwargs)

# using requests-futures

print('requests-futures:')

session = CustomSession()

futures = []
for i in range(10):

    futures.append(session.get('http://google.com/', test_id=i))
for future in futures:
    try:
        r = future.result()
        #print((session.timing[i]['cE'] - session.timing[i]['cS']))
    except Exception as e:
        print(e)
for i in range(10):
    print((session.timing[i]['cE'] - session.timing[i]['cS']).total_seconds() * 1000)


# using requests

print('requests:')

for i in range(10):

    check_start_timestamp = datetime.utcnow()
    r = requests.get('http://google.com')
    check_end_timestamp = datetime.utcnow()
    cE = int((check_end_timestamp - check_start_timestamp).total_seconds() * 1000)
    print(cE)

      

        
        
        
      

    

futures requests:

112.959
118.627
160.139
174.32
214.399
224.295
267.557
276.582
316.824
327.00800000000004

      

        
        
        
      

    

requests:

99
104
92
110
100
126
140
112
102
107

      

        
        
        
      

    

It seems that

• Response time s requests-futures
  
  appears as additive (time increases)
• Using simple requests
  
  is much faster.

This is normal? Am I missing something that might cause a discrepancy?