When I prepare my raised beds in the spring and in the fall, they are each given the same amount of compost and new soil. They each receive the same amount of moisture and sunlight.
I planted eight hybrid cucumber seeds from the same packet along an eight foot long trellis about three weeks ago. All of the seeds germinated within twenty four hours of each other. One plant has grown up the trellis to about 36" in height. The other seven plants, while very healthy; are only about 18" tall.
I planted two beds in yellow crook neck squash in the spring and replanted for a fall crop. One bed produces very healthy, but smaller plants which produce squash. The other bed produces much larger plants in the same amount of time and the squash are significantly larger. All of the squash seed came from the same packet.
In the case of the cucumbers, it seems one seed was simply genetically stronger than the other seven seeds. In the case of the squash, the difference must be in the beds, but I can't imagine what it would be.