This is what made me stop and think. On the Distrowatch 'page hit ranking' chart (which is a reasonable indication of each distro's popularity) Ubuntu has slipped from the top within the past 3 months.
Read the comment at seopher.com
Why could this be?
Well, PCLOS has not had any new releases lately; the last one was the distribution release back in May (the 21st if memory serves) so it's not traffic relating to a new product. Ubuntu has a new release out in October (7.10) and put out development release Alpha 4 on the 9th of this month.
Logic would dictate that even if they carried identical popularity then Ubuntu would still have a higher ranking due to the interest around the Alpha releases. Not so. PCLOS has been receiving 400-500 hits per day higher than Ubuntu.