When using Magneto with stereo output (both LEFT and RIGHT OUTPUTS connected), there is a psycho acoustic process applied to the output signal when the panning is centered on Magneto to provide a sense of depth to the audio.
In this process, we start by adding a very slight delay to the LEFT OUTPUT channel. Because of this slight delay, the RIGHT, non-delayed OUTPUT channel is perceived as louder to the human ear because it is heard first, so we boost the level of the delayed output very slightly to compensate for this and provide a sense of space between the LEFT and RIGHT output channels.
When using only a single output from Magneto, output signal sums to mono* and the psycho-acoustic process is not applied as there is no interaction with the LEFT and RIGHT outputs. Also, when set to CENTER PAN with both LEFT and RIGHT OUTPUTS connected, as soon as you adjust any of the heads to a custom panning setting, the psycho-acoustic process is removed.
*The summing to mono only applies when the PAN toggle switch is set to LRLR, LRRL, or using a custom PAN setting in CENTER and occurs before the psycho-acoustic process.