Let's answer your question in two parts.

For the first part of your question, in Lois Lowry's The Giver, it seems the idea of "Sameness" comes from an idea of "Non-Sameness" in the world that preceded the one the protagonist, Jonas, grows up in. In The Giver, it is implied that the Elders of Lowry's dystopian world reconfigured society based upon their conclusion that "Non-Sameness" resulted in a negative and detrimental life for individuals. In other words, the Elders' interpretation of the past formed what we may term a "system of Sameness." Of course, the extent, or complete lack thereof, to which the Elders' interpretation is honest or accurate is a subject worthy of continued discussion, but we won't go into that here.

Now, for the second part of your question, there are differences between members of the community in Lowry's novel other than those of gender and assigned jobs. For example, each member of the community has particular characteristics different from those of others.

The people of the community were trying to eliminate sources of unhappiness. They believed that if no one had any more or less than another, had the same amount of love for every person, then everyone would be equal and therefore equally happy. They didn't see the need for the advances that come from competition between people of varying abilities and with different interests; as far as they were concerned, things were perfect the way they were, and there was no need for any change.