They are a few leftovers from the original batch. Calvin only has 5 or 6 of each and only has 6 of the series (Jupiter and Earth are not available).
I have 2 of the black ones and having solved Jupiter with ease, will make an Earth out of the black and have ordered the other 6 as soon as they arrived on the store.
The present wife was none to happy to hear that yet more cubes are on their way. But what the hell! Wives can easily be replaced - these cubes will never come around again!