This seems status-bydesign. Based on the current time of 2014-09-10 14:31:43Z,
"today" is defined to be the time range 2014-09-09 14:31:44Z to 2014-09-10 14:31:43Z (or the past 24 hours) and is listed in a "just now", "Xs ago", "X mins ago" or "X hours ago" fashion;
"yesterday" is defined to be the time range 2014-09-08 14:31:44Z to 2014-09-09 14:31:43Z (or between 48-24 hours ago);
times prior to 2014-09-08 14:31:44Z (or more than 48 hours ago) will be listed in a "X days ago" fashion.
The time stamp 2014-09-08 19:25:37Z falls within the "yesterday" category.
As reference, the current UTC time can be obtained by clicking on Achievements in the Top Bar:
