Going from six to seven billion people is a 16% growth whereas a lot of the statistics show much larger changes so even before factoring population the figures are probably still significant. There are nuances such as the exponential effects of the internet and mobile phones (a mobile phone or internet connection can have a massive impact on a remote community) and other unknown factors. If you dive into the academic literature I'm sure you can find some analysis of the figures.

I believe the extreme poverty goal used absolute numbers. Percentiles wouldn't make sense to begin with because extreme poverty is largely inherited and as you said could be dwarfed by population growth.