Our ears use different methods of hearing at different frequencies to allow us to localise sound. We use phase differences at low frequencies and level/intensity differences at high frequency (a slight simplification but it’ll do). Traditional amplitude panning actually recreates these low frequency cues if you’re sitting in the sweet spot i.e. equidistant from both loudspeakers.
But there’s a problem: using the same amplitude across the whole frequency spectrum means that the cues for the low frequency tell your brain that the image is one one direction and the high frequency cues tell it that it’s in another direction.
A very smart man (which is underselling him) called Michael Gerzon developed two vectors, known as the Velocity and Energy vectors, that can give you an idea of where a sound will be perceived if you know the loudspeaker gains. The Velocity vector is an indication for low frequencies below approximately 700 Hz and Energy vector for high frequencies. If we consider the prediction a standard stereo set-up (2 loudspeakers and a listener forming an equilateral triangle) shown to the right then there is indeed a discrepancy between the directions of the two Gerzon vectors!