It probably changed on July 1. Here is some more info: http://www.ets.org/gre/subject/scores/understand/
And also: http://www.ets.org/s/gre/pdf/gre_guide_table2.pdf
This PDF shows how percentile rank are converted from your scaled scores. The text at the top says that this file shows the conversion for all exams taken between July 1 2009 through June 30 2012 (i.e. a 3 year period). I would guess that your percentile rank are recomputed every July 1 compared to scores of all exams taken in the past 3 years (i.e. rolling window). Thus, if you took the April 2013 test, when you got your score in May/June 2013, your percentile rank was computed based on the July 1, 2009 to June 30, 2012 window. However, today, when you check online, it's likely that your new percentile rank is computed based on the July 1, 2010 to June 30, 2013 window.
If this is right, then your scaled score should never change (that score is specific to the test you wrote on your particular exam date). But, your percentile rank is updated every year because it compares your performance on your specific test to the performance of everyone on the past 3 years worth of tests.