Thanks for posting this, in this case this is the intended solution. The extra box is there for a few reasons, but it basically comes down to appearance and making the solution slightly less telegraphed. With five boxes it is somewhat easy to see where each one needs to end up and then reverse-engineer the solution (a tactic I would not usually discourage, but I felt it undermined the satisfaction of finding the "trick" here); with six the idea was that it would feel like too many so the player wouldn't know how many they actually needed to use. It ended up being more jarring than I intended, especially when the previous 43 levels used pretty much everything provided.