The other side of the fence

I always heard the grass is always greener on the other side of the fence.  So, how does that apply tot he ripening of fruit on a plant?

I stopped outside our front gate to take this photo.  The closer cactus, the one with the green fruit, is outside of the fence.  It gets a little less shade, and about the same amount of water.  All of its fruit are green, whereas the cactus on the inside of the fence has all of its fruit is ripening stages from almost ripe to completely ripe.  

The one outside the fence began as a start from the one inside the fence, but has pointy shaped pads as though it is a different type of prickly pear.  Hmmmm... think thee might be some missing information as to why, but is the fruit greener on the other side of the fence?  I love pondering this stuff with the kids, because it leads to so many possibilities.  Wyatt usually figured the light and water, Bailey starts thinking genus and species and crosses.  What do you think?