This is just RNG.
It works based on weightings (regardless of how many different artefacts a spot contains), and we track the last three artefacts received. If you were to three of A in a row, your weightings for A/B/C would be 1/10/10, so still have a 1/21 chance to receive A again. It happens.
We have discussed increasing the prior artefact tracking from three to five, which should help some more.