Selenium test failed

I'm learning Selenium. Why this simple test is failing?

from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions
from selenium.webdriver.common.action_chains import ActionChains
import unittest


class ToolTipTest (unittest.TestCase):
    def setUp(self):
        self.driver = webdriver.Firefox()
        self.driver.get("http://jqueryui.com/tooltip/")
        self.driver.implicitly_wait(30)
        self.driver.maximize_window()

    def test_tool_tip(self):
        driver = self.driver
        frame_elm = driver.find_element_by_class_name("demo-frame")
        driver.switch_to.frame(frame_elm)
        age_field = driver.find_element_by_id("age")
        ActionChains(self.driver).move_to_element(age_field).perform()
        tool_tip_elm = WebDriverWait(self.driver, 10).until(
            expected_conditions.visibility_of_element_located(
            (By.CLASS_NAME, "ui-tooltip-content")))

        # verify tooltip message
        self.assertEqual(
            "We ask for your age only for statistical purposes.",
            tool_tip_elm.text)

    def tearDown(self):
        self.driver.close()

if __name__ == "__main__":
    unittest.main(verbosity=2)

Sometimes it gives

AssertionError: 'We ask for your age only for statistical purposes.' != "ThemeRoller: jQuery UI's theme builder app
lication"
- We ask for your age only for statistical purposes.
+ ThemeRoller: jQuery UI's theme builder application

Sometimes:

AssertionError: 'We ask for your age only for statistical purposes.' != "That's what this widget is"
- We ask for your age only for statistical purposes.
+ That's what this widget is

So the error isn't always the same. It looks that it select a random popup. Here's the page to test. I'm using python 3.6 selenium 3