Backlinks grouping in the New/Lost Backlinks report depends on the selected mode.
Backlinks are not grouped.
The calendar shows for each date how many new backlinks were found, and how many backlinks were lost.
One link per domain
Backlinks are grouped by the root domain of referring pages. Each group shows the "top" backlink with the highest URL Rating first.
If one of backlinks in a group falls into a range selected in the calendar, the whole group is shown as new and lost on the respective new and lost dates of the "top" backlink that falls into the selected range. If every backlink in a group falls outside of the selected range, it is shown as new and lost on the respective new and lost dates of the "top" backlink in the group.
Hence, selecting a different range in the calendar, or changing the report between showing new or lost backlinks will change many numbers in the calendar.
Group similar links
Backlinks are grouped by the root domain, anchor text, and text surrounding the anchor text of referring pages.
The calendar shows for each date, depending on the mode:
how many new groups appeared on that particular date. "new group" means a group that has never been seen before;
how many lost groups disappeared on that particular date. "lost group" means a group that lost all its backlinks.
Because of the definitions of the "new" and "lost" groups, selecting a different range in the calendar does not affect grouping, and it does not affect numbers in the calendar.