This is because at zero power mismatch, when islanding occurs, there is no noticeable deviation in frequency and voltage. Hence, the measure of voltage and frequency wont indicate island detection. So we have to opt for other methods which primarily focuses on other parameters. Therefore, to proof that a particular method works (detects island) accurately under perfectly balanced voltage and frequency conditions (i.e. zero non-detection zone), we detect Islanding in multiple DG system when ever approximately zero active/reactive power mismatch.

You can follow the following two papers to get a more detailed idea.

Article Smart Inadvertent Islanding Detection employing p-type μPMU ...

Article Shifting of Research Trends in Islanding Detection Method - ...

In other words, its very tough to detect islanding using conventional techniques (such as over/under voltage, frequency, etc) when there is no mismatch in the island. Also, (unintentional) islanding needs to be detected so that the microgrid owner can take necessary precautions, even if there is zero mismatch. Just because there is no mismatch now, does not mean it will stay this way while the system is islanded.

Islanding is not able detect in NDZ region.Especially in Multiple DG's ,if islanding is happened,the remaining DG's gets overloaded and there is possibility for cascade tripping. Thats why it is necessary to detect islanding incase of multiple DG'S.