The mushroom houses at the start of each world (the point with an arrow at the start) differ determining on the time you get on a stage.

If you get x11, x22, x33 on the timer, a Red Mushroom house appears, which generates items.
If you get x44, x55, x66 on the timer, a Green Mushroom house appears, which is a 1up minigame.
If you get x77, x88, x99 on the timer, a Orange Mushroom house appears, which lets you gain a Mega Mushroom into your inventory, thus overriding anything in it.