What I believe happened is that the Walmart clerk found a chart that shows what the non-resident fees are in each state. Apparently, Maryland DNR includes this chart to show how low their fees are compared to most other states. The clerk interpreted this chart as the list of fees to be charged to those in other states.
I contacted DNR and they confirmed that the fees were in fact $5 and $7.50 for 3-day and 7-day non-resident licenses. They also included a comment to the effect of "typical Walmart clerk behavior".