In 2007 DST changed and Mexico did not follow suit, so the short PST/EST/CST timezones were not specific enough anymore. Here is an excerpt from Wikipedia (http://en.wikipedia.org/wiki/Pacific_Time_Zone):

Effective in the US in 2007 as a result of the Energy Policy Act of 2005, the local time changes from PST to PDT at 02:00 LST to 03:00 LDT on the second Sunday in March and the time returns at 02:00 LDT to 01:00 LST on the first Sunday in November. Canada would also acknowledge this change. In Mexico, beginning in 2010, the portion of the country in this time zone uses the extended dates, as do some other parts. The vast majority of Mexico, however, still uses the old dates.