The exact same question has been asked before
. . . but my previous guess was definitely wrong . . .
So let me upgrade my guess . . .
Sending a command between 1500 to 1900usec rotates the servo from 90 degrees to 180 degrees. And 1900-1500=400usec, with 90 degree travel. Neutral position being 1500usec.
Now servocity.com thinks that servos can only rotate a total of 90 degrees, for 45 travel. For example, this pic for 180 degree servos on their site is obviously wrong causing constant confusion on this forum:
So when they wrote "45 Deg. one side pulse traveling 400usec", I think they meant to say "90 Deg. one side pulse traveling 400usec from neutral".
Or in easy to understand terms, send it 1900usec to rotate to 180 degrees, 1500 for 90 degrees, and 1100 for 0 degrees.