The higher numbers represent the actual number assigned to that channel. Mapdowns tell boxes that the channel can be found in a more convenient location for the customer.
There are a lot of older boxes which are not capable of mapping down channels. For those customers, they would have to go to the actual channel number. A model 3700 receiver won't go to a channel 16 in Scranton, a customer with that box would have to go to 9247 or wherever the actual channel is located.
HD mappings are changed because the SD channels are located at their native spot for all receivers, including legacy receivers. So a 3700 model receiver will see History at 120 because that's where DISH tells it to find it at. In the case of HD History, if they were to uplink that as 120, it would screw up every receiver in the system, so they uplink it as a 94xx instead. Telling the boxes to map it to 120 is just a convenience thing for the customer.
COX here in Phoenix puts their HD locals in the 700s so 703 is the HD version of channel 3. However, many cable HD customers wouldn't remember to go to the 700s for their HD locals and would end up complaining about poor picture quality on their HD locals because they were going to the SD ones instead. When I moved out here and worked a temporary gig doing cable, I was teaching people constantly where to find their HDs since they weren't mapped to be co-located to the SD counterparts. DISH makes it easy by defaulting you to the HD version of a channel when you type in that number.