I do not work for UPS, but I did at one time from 1992 to 1999. Met my wife there.
Ground is always by ground and they can do that since there is no guarantee on the delivery date (just an estimate). But ground can also mean TOFC (Trailer on Flat Car) which is trailers loaded on rail cars. Worldport gets the biggest recognition, but outside of Chicago, UPS has a major ground/rail operation. Anything ground headed from one coast to another goes through there. It's called the UPS Chicago Area Consolidation Hub (or CACH).
It takes a ground package about 2-3 days to get through CACH. Primarily, that's due to scheduling and assembling trains by BNSF whose rail yard is connected to CACH.