Flummoxed neighbor demands to swap sides of shared garden when she sees that couple's plants are growing better than hers: '[We] spent a lot of money on new fresh soil, plant food, everything. Spent countless hours tilling and making it great for growing'

To a lot of people, a patch of dirt is a patch of dirt, but to a gardener, the dirt is so much more than that. A good garden takes constant management and not just in watering the plants. Ensuring the soil contains the correct precious nutrients is, quite arguably, the most important part of the entire process. Some who seek to grow plants and crops without understanding this will be flummoxed as they futilely toil in soil that is never going to reward their efforts. 

The problem is that this landlord is one of those "it's a patch of dirt" people and can't possibly understand what all the hoopla over some dirt is about, why one tenant is insisting that the sunlight is different on one side, or why the other tenant flat-out refuses to swap. To him, it's a he said she said situation, and there is an inherent problem is his lack of knowledge or experience in the case he is the judge of.

This neighbor clearly hasn't got a clue either, if they think that the only thing that could cause a difference is sunlight. They're correct that there seems to be an observable, correlative difference in how the plants in both locations are growing. But, correlative is not causative, like in so many aspects of life these days, they've gone full Dunning-Kruger and mistook their uninformed position for an informed one and what is essentially their opinion for a verifiable fact. There also seems to be some coping going on here by the neighbor in blaming an external factor for the couple's relative success rather than on something that the kids these days would call a "Skill issue."