I’d say this product was reprocessed a year after it was sensed, probably due to some error identified. We currently keep all the products in the archive, not deleting the old ones as it is not always clear what is the best action.

But these are exceptions, rather than the rule.

Note that with Sentinel Collection 1 mass processing happening by ESA as we speak, these anomalies will clear out. After reprocessing is fully finished, we will probably delete the old products.