It helped me to think about it in terms of connected graphs. Nodes are versions and arcs are changes. A path between two versions is the aggregation of changes. Merging then becomes walking down the same path, but starting from a different node. I start at my current version node, and I walk down your path of changes. It's very different than trying to merge two nodes.

(This is just my mental model, no need to read into it more than that)