But the DTVPal and CM-7000 converter boxes
(not clones of each other, unlike the DVRs) also allow 3-digit channel numbers. No smart card slots or Internet connections on them.
Maybe the DTVPal CECB allowed 3-digit channels for consistency with the (then upcoming) DVR. But the CM-7000 CECB was completely unrelated.
I think ATSC theoretically allows for 3-digit channel numbers, so Dish and CM provided for them. Even though, even if 3-digit channel numbers were allowed, no station would use them since, aside from Dish and CM, most ATSC receivers don't support them. This is the sort of thing that can happen when you have a product designed "to specs" in a part of the world that doesn't even have ATSC stations.
For Dish, another possibility is the way stations with duplicate channel numbers are handled. (These are rare, but can happen.) Only one station gets its correct number: the other(s) get dummy channel numbers starting at 70. Perhaps Dish provided an extra digit in case there were over 30 duplicates somewhere!