As noted, there was recently some confusion after The Wrestling Observer reported after SummerSlam week that NXT Takeover topped WWE SummerSlam as the most watched show of the week on the WWE Network. I noted days later that those rankings use an algorithm that takes into account video-on-demand (VOD) and when they were added, and they are not based on total views.
Dave Meltzer noted on Wrestling Observer Radio recently that he spoke to someone with WWE, and they confirmed that the rankings are based on VOD views, and they don't count the views of the live stream. So the reason NXT: Takeover was ranked higher for a couple of days was because more people watched it on demand, whereas more people watched SummerSlam live.
SummerSlam eventually passed Takeover in VOD views that week. In the most recent list of the most popular shows on the Network, SummerSlam was still number one, followed by last week's episode of Cruiserweight Classic and last week's NXT.