Today measure command only support 100 distinct results. It´s a risk that alerts created with measure command don´t give correct results because of this limit. Now the first top 100 results is sent to measure.

From documentation:

Second, Measure count currently returns only the top 100 distinct results. This limit does not apply to the other statistical functions. So, you'll usually need to use a more precise filter first to search for specific items before you apply measure count().

https://docs.microsoft.com/en-us/azure/log-analytics/log-analytics-log-searches#use-the-measure-command