Why does material build up on spot welding tips?

During spot welding because the two pieces of metal melt together at the point of highest resistance to current flow, basically when the electricity flows through the pieces it heats up at the point of highest resistance that is where the metal pieces touch and in a direct line which makes the electrode melt resulting in material build up at the tips.

Because this point have the largest electrical resistance and melting occurs in the separative area between2 sheets @ this poit