In long run, it is off course the house edge but in short small sessions it is mainly variance. You give me 45 hits of an EC in any scatter you feel like, in 100 spins. House edge and a slight negative variance both will be there in 45/100 but it can be beaten with 1 unit of profit but if it is 35/100 or 30/100, it maybe impossible to earn 1 unit in a variety of scatter and clusters of wins and losses.

So, in small playable session, it is mainly down to positive or negative variance to win or lose and not the house edge. A very naive player can lose in 100 spins, even with 55/100, if he is not prudent enough to not to use martingale, fibonacci or labouchere kind of deadly progressions that can finish the bankroll in a short successive run of losses. Again, the reason of loss of that player will not be house edge but variance. In long run, the tilt of house edge will leave no room for coincidences and will bring in horrible variance at some point of time to kill the player.