Beginning with May 1, 2019, many conversion metrics in the attribution reports in Google Ads are calculated differently. (See "What Changed" below.) The calculation change was not applied retroactively, so affected metrics use one calculation for days prior to May 1, 2019 and another calculation for days beginning with May 1, 2019.
If your active date range extends before and after May 1, 2019, your conversion metrics will be based on two different calculations—one calculation for the days prior to May 1, 2019, and another calculation for subsequent days beginning with May 1, 2019.
Beginning with May 1, 2019, all conversion metrics (e.g., "Conversions," "Conv. value," "Last click conversions," etc.) in attribution reports include cross-device activity. However, for days prior to May 1, 2019, cross-device activity was only included when "Device" was explicitly selected as the pivot dimension.
Why this change was made
Prior to May 1, 2019, the former "Devices," "Assisting Devices," and "Device Paths" reports included cross-device activity in their calculations, while the other reports did not. As a result, the conversion counts were inconsistent across different reports. To address this inconsistency, all conversion metrics now include cross-device activity, beginning on May 1, 2019. However, for days prior to May 1, 2019, this inconsistency still exists.