The sunrise and sunset calculations in MyAlmanac are based on the definition that the day starts or ends when the top of the disk of the Sun touches the horizon. This is roughly when the center of the sun is 0.83 degrees below the horizon.

On the other hand. the claim that night and day are symmetric (i.e., last equally long at the equinoxes) is based on the definition that the day starts or ends when the middle of the disk of the Sun touches the horizon.