the rotors generate a back-current out of phase with the incoming current
and being the two motors are not balanced to each other, the back EMF from one motor is helping to feed the other. The larger the motors the greater the effect. Most people do not notice it on the small motors or do not get to excited about an amp or so.

We can have a 30 and a 40 hp running and then run a 10 almost for free. Well we could if the power company did not have a "special" meter that is supposed to correct for this.

Lentz's law kinda sorta helps to explain it too.