Complex sentence: That has caused as well the report on which the Top 25 is based to lack that data--causing a significant under count in total page views, as mobile views now account for about 30% of Wikipedia traffic.
Split into simpler sentences: That has caused as well the report on which the Top 25 is based to lack that data. This has caused a significant under count in total page views, as mobile views now account for about 30% of Wikipedia traffic.