The acquisition report shows a total number of users in the first column, broken down by channel. The number of users in each channel is followed by a percentage. This percentage does not reflect the actual percentage of (users in channel) / (total users). The percentage shown in consistently lower, suggesting a larger number of total users. I have seen this same discrepancy across all of my GA implementations, as well as in screenshots on the web of this report. What is this number a percentage of?
Strangely, the percentages shown for New Users and Sessions in the same report are equal to (new users or sessions in channel) / (total new users or sessions). I wondered if this had to do with how new vs. returning users are counted, but adding new and returning users in the Audience reports does not result in the same number.