Possession-adjusted (styled as PAdj or Opp30) is a method to calculate defensive statistics to take possession values into account.
While the average duration of the match is slighly on rise every year and is at 95 minutes 30 seconds for 2019, the pure time of ball in play is in decrease and is generally between 50 and 60 minutes depending on the league. Wyscout took 60 minutes as a reasonable value to adjust possession-dependent stats to.
The logic for adjusting defensive statistics is the following: you can only make defensive contribution when you're not in the possession of the ball. Therefore, when you look for high defensive values, normally you would only encounter the defenders of the lower teams in the league: their defenders, being dominated by a possession-leading team, are forced to make more actions (defensive duels, interception, sliding tackles etc.). The defenders of possession-based teams are naturally making less actions. Adjusting these values to the possession (as if the match was played with a 50%/50% possession) gives further insight to the frequency of defensive actions.
In the match of AFC Bournemouth - Manchester City (0:1, 2 March 2019) Man City had had 80% of possession (42:37 pure possession time), while Bournemouth only had 20% (10:35 pure possession time).
Aké, who played the whole match for Bournemouth, had 10 interceptions. His PAdj interceptions value would be: 10 / 42.5 * 30 = 7.06.
Walker, who played the whole match for Man City, had 5 interceptions. His PAdj interceptions value would be: 5 / 10.5 * 30 = 14.3.
Therefore, while Walker made only half of interceptions of Aké, the huge difference in possession make his possession-adjusted value twice higher.