Here's another quirk. You have new product that you want to feature in your e-mail targeting strategy.
If the has never been sold, you probably cannot come up with a weighting score for this product. In this case, you make up manual rules for deciding who gets that version of the e-mail campaign.
But if the product has been available for a few months, you may get a decent weighting score. In these cases, you may need to artificially increase the weights, in order to allow enough customers to receive this version of the e-mail campaign (multiple versions on the same day, customer only receives one version on that day).