Tiempos de carga de la página en firebug y google analytics

I understand that average page load speeds recorder by google analytics will differ from speeds recorded on my machine with firebug, that's pretty obvious. It's an average of page loads on different machines, browsers and broadband speeds

However, how come google report would show that homepage takes longer to load than product page where in firebug is the opposite?

What's your thoughts on monitoring web sites speeds in general using one of those (or both) tools?