Assassins and unit placement

So can anyone explains me how this work? When I play assassins, some of the rounds the enemy units doesn't move an inch and my assassins jump onto their frontline and get obligerated. Other rounds when another player has units placed in exact same way his units will move towards my tank placed in middle bottom (and assassins are placed on both sides of this tank) which create gaps the assassins can jump into and my assassins will obligerate enemy units.

I don't see consistency in that. It is not like enemies were swapping carries into front and putting tanks behind them (that won't work at all) they have melee tanks in front but they just don't move sometimes.