The blocks in Peru are not melted. They are simply shaped to remove as little stone from the block as possible which can give them more than six sides. And the exterior of the wall is not representative of the whole wall construction. The close fittedness of the blocks only extends for a few centimeters into the wall. After that, large spaces and openings form which were packed with earth and rubble.

For a more detailed analysis on Inca stonemasonry, please check out this topic I wrote over at /r/badhistory.