I think we may be complicating this. You just need a count of CustomerIDs per month per title. No Running total is necessary.
in your crosstab keep your colums as the date and your row as the Title but change your Summarized field to the CustomerID and summarize it as a Count or a DistinctCount. A count will include it more than once if it can appear more than ionce in these groupings. A DistinctCount will oply count it once per grouping even if it appears more than once.