Inconsistency Between Unique Users and Cumulative Users in Insights Reports Explained

I'm seeing an inconsistency in unique users vs cumulative users on insights reports. On day 1, there were 91 unique users of group a, day 2 had 88 users. On the cumulative users report, day 1 shows 91, but day 2 shows 143. Neither report has any filters, and the metric and breakdown and time period and time of data query are the same, and users are counted in every segment they appear in for the cumulative users report. How is this possible?