Phase is always problematic for volume, a phaser mixes two versions of the original signal with each other, one with a variable frequency phase shift. When the two signals have the same amplitude a particular frequency is exactly in phase, the original's amplitude is doubled or if out of phase it is totally cancelled out. Essentially it's applying frequency-dependent boost and cut, this means that the before and after volume depends both on the frequency spectrum of the original signal and with time as the phase shift varies.
In other words it's inherently difficult to match, which makes it remarkable how my ancient 'Melos Mini Phaser' that just has a speed control does so well!