I'm an American on Eastern time and I work pretty routinely with people in other US time zones. I get this pretty much all the time. I think people think that putting the "Standard" in makes it sound more "official".
There is very little understanding in the US that "Daylight" time stops being "Standard" time. (because why should it? why have two/three time zones when you only need one?)
Problem is that there aren't any widely-accepted acronyms for time zones in the United States that don't include the current daylight savings time status. Eastern is way longer than EDT.