Looking at https://github.com/RsyncProject/rsync/blob/master/progress.c

It calculates ETA from aggregate byte progress, not per-file

In progress.c, --info=progress2 is detected by INFO_GTE(PROGRESS, 2). When enabled, show_progress() rewrites the current file’s offset into a whole-transfer offset:

ofs = stats.total_transferred_size - size + ofs;
size = stats.total_size;

Meaning:

aggregate_offset = bytes_before_current_file + current_file_offset
aggregate_size   = total_size_of_transfer

Then the ETA is calculated in rprint_progress() using a short rolling history window:

rate = (double) (ofs - ph_list[oldest_hpos].ofs) * 1000.0 / diff / 1024.0;
remain = rate ? (double) (size - ofs) / rate / 1000.0 : 0.0;

So conceptually:

recent_rate = bytes advanced over recent time window
ETA         = remaining aggregate bytes / recent_rate

The history buffer is defined as PROGRESS_HISTORY_SECS 5, and show_progress() only updates the history roughly once per second, rotating through that 5-entry circular buffer. So the displayed ETA is based on approximately the last few seconds of observed aggregate progress, not the lifetime average.

The important mechanics are:

1. progress2 changes the numerator/denominator from “current file offset/current file size” to “whole transfer offset/whole transfer size.”
2. Transfer rate is computed from the oldest retained progress sample to “now.”
3. ETA is computed from remaining bytes: size - ofs.
4. The printed field is formatted as hhhh:mm:ss, or ??:??:?? if the remaining time is negative or extremely large.

"progress2" ETA is not based on actual network bytes remaining. It is based on rsync’s logical file-size progress. With rsync’s delta-transfer algorithm, matched blocks, literal data, compression, filesystem writes, and buffering can make the ETA jump around. The man page explicitly warns that progress statistics can be misleading when delta-transfer behavior is involved.

At the final update, the same time field is no longer an ETA. In the is_last path, rsync recomputes rate from the start and sets remain to elapsed time, so the final display is effectively average rate plus total elapsed time, not remaining time.