Percy calculates your usage based on the number of snapshots/screenshots you are trying to take per build.
As an example, if you snapshot one page on:
The above would result in 6 snapshots (i.e., 1 snapshot x 3 widths per browser).
Along with this, the usage would include the failed build attempts (as Percy did open a browser and tried to capture a snapshot; however, the snapshot wasn’t captured due to an exception).
Hence, the total snapshots would be calculated as a sum of successful and unsuccessful snapshots.