Difficulty trying to use flags to move button widget in wxPython

I am having some difficulty placing 2 buttons centered in the bottom using flags. The output is this when I try, and I don't understand why they don't align to the bottom center.:

With the following code:

#!/usr/bin/python

# gui.py

import wx
import webbrowser

class PanelOne(wx.Panel):
    def __init__(self, parent):
        wx.Panel.__init__(self, parent=parent)

class PanelTwo(wx.Panel):
    def __init__(self, parent):
        wx.Panel.__init__(self, parent=parent)

class GUI(wx.Frame):
    def __init__(self, *args, **kwargs):
        super(GUI, self).__init__(*args, **kwargs)

        self.poo()

    def poo(self):
        self.CreateStatusBar()
        menubar = wx.MenuBar()
        fileMenu = wx.Menu()
        self.add_entry = wx.MenuItem(fileMenu, -1, '&Add Entry\tShift+A', 'Add employee')
        quit = wx.MenuItem(fileMenu, wx.ID_EXIT, '&Quit\tEsc', 'Quit application')
        fileMenu.AppendItem(self.add_entry)
        fileMenu.AppendItem(quit)
        menubar.Append(fileMenu, '&Options')
        self.SetMenuBar(menubar)

        self.panel_one = PanelOne(self)
        self.panel_two = PanelTwo(self)
        self.panel_two.Hide()

        vbox = wx.BoxSizer(wx.VERTICAL)
        vbox.Add(self.panel_one, 1, wx.EXPAND)
        vbox.Add(self.panel_two, 1, wx.EXPAND)
        self.SetSizer(vbox)

        pv1box = wx.BoxSizer(wx.HORIZONTAL)
        self.panel_one.SetSizer(pv1box)

        self.chrome_button = wx.Button(self.panel_one, id=2, label='Gmail')
        self.outlook_button = wx.Button(self.panel_one, id=1, label='Outlook')

        pv1box.Add(self.chrome_button, flag=wx.ALIGN_BOTTOM | wx.ALIGN_CENTER | wx.ALIGN_RIGHT)
        pv1box.Add(self.outlook_button, flag=wx.ALIGN_BOTTOM | wx.ALIGN_CENTER | wx.ALIGN_RIGHT)


        self.SetSize((450, 300))
        self.SetTitle('Prototype')
        self.Centre()
        self.Show(True)


        self.Bind(wx.EVT_MENU, self.onQuit, quit)
        self.Bind(wx.EVT_KEY_DOWN, self.onESCQuit, quit)
        self.Bind(wx.EVT_MENU, self.onSwitchPanels, self.add_entry)
        self.chrome_button.Bind(wx.EVT_BUTTON, self.onClick)
        self.outlook_button.Bind(wx.EVT_BUTTON, self.onClick)


    def onQuit(self, e):

        dial = wx.MessageDialog(None, 'Are you sure to quit?', 'Question', wx.YES_NO | wx.NO_DEFAULT | wx.ICON_QUESTION)
        input = dial.ShowModal()

        if input == wx.ID_YES:
            self.Close()

    def onESCQuit(self, e):
        esc = e.GetKey()

        if esc == wx.WXK_ESCAPE:
            dial = wx.MessageDialog(None, 'Are you sure to quit?', 'Question', wx.YES_NO | wx.NO_DEFAULT | wx.ICON_QUESTION)
            input = dial.ShowModal()

            if input == wx.ID_YES:
                self.Close()

    def onSwitchPanels(self, event):
        if self.panel_one.IsShown():
            self.add_entry.SetText('Notifications')
            self.panel_one.Hide()
            self.panel_two.Show()
        else:
            self.add_entry.SetText('Add Entry')
            self.panel_one.Show()
            self.panel_two.Hide()

    def onClick(self, event):
        if event.Id == self.chrome_button.Id: 

            chrome_path = 'C:/Program Files (x86)/Google/Chrome/Application/chrome.exe %s'
            # Without this path it will open in Internet Explorer
            webbrowser.get(chrome_path).open('https://mail.google.com')
        else:
            chrome_path = 'C:/Program Files (x86)/Google/Chrome/Application/chrome.exe %s'
            # Without this path it will open in Internet Explorer
            webbrowser.get(chrome_path).open('https://outlook.live.com')


def main():

    app = wx.App()
    GUI(None)
    app.MainLoop()

if __name__ == '__main__':
    main()