In sj-extracting.c, on lines 308-309, before extracting each track the progress bar is updated thusly:

/* Update the progress bars */
gtk_progress_bar_set_fraction (GTK_PROGRESS_BAR (progress_bar), CLAMP (1.0 - ((g_list_length (pending) + 1) / (float)total_extracting), 0.0, 1.0));

This sets the progress bar to the percentage of tracks extracted.  However, while in the middle of extracting tracks, the progress bar is set to the percentage of TIME DURATION extracted.  See line 383-385:

float percent;
percent = CLAMP ((float)(current_duration + seconds) / (float)total_duration, 0, 1);
gtk_progress_bar_set_fraction (GTK_PROGRESS_BAR (progress_bar), percent);

These two percentages are clearly not the same, and differ more the greater variance in track lengths.  For example, ripping a very short song increases the % of tracks extracted much more than the % of time duration extracted.  So, depending on the CD, the progress bar can swing wildly back and forth as it completes each track.  Since current_duration and total_duration are already properly defined where the first progress bar is used, they should be used there as well.  Since intra-track progress can't be measured any other way-- unless you want something more complicated, like % of tracks done + (current track %)/(# of tracks).