If market share is measured in dollars, the difference could wholly be from Walgreens selling more high-cost branded prescriptions while competitors sell lower price generics. In such case, the WAG rise in market share is a defacto loss in the number of prescriptions filled.
If so, this exposes yet another Achilles heel in WAG's marketing and business plan. Needs further research to validate.
On the bright side, there is a market for pork rinds in Puerto Rico.