Had the same problem with a missing Walmart in Kingsland, GA and missing post offices (3 of them!) in the Tampa area. Some of that POI data is just flat wrong, whether it came on the GPS or was added from sources like POI Factory.
One source of errors are the multiple schemes for writing lat/long coordinates. If coordinates are entered in one notation format but the GPS is using another, you will be taken to the wrong place every time, no matter whose map is inside.
Formats I know of include:
Degrees+minutes+seconds (traditional) xx degrees, yy' zz"
Degrees+minutes.decimal minutes xx degrees yy.yyyy'
Degrees.decimal degrees (all decimal format) xx.xxxxx degrees