It was discovered that there is a bug in the MailChimp API that makes it so that requests for the "List Growth History" (aka the Existing Members field) may return 0 values or blank values for some months. The issue was confirmed here by MailChimp Support but it does not appear to be addressed:
Note that we do not support the work around mentioned in the MailChimp response yet, as we don't provide segments, but this a feature already in our backlog to be added at a later point.
We apologize that this API bug prevents our products from pulling the correct information for your list growth. If you have further questions about the issue, please reach out to MailChimp support for assistance.