Please pay more attention to the point 3 in my original post. To reiterate: their encoding is hilariously bad, and is easily outcompeted by a modem from the 60s.
on the subject of the encoding efficiency, the ggwave depo mentions the use of reed-solomon error correction to make transmission more reliable. im struggling to find any info on error correction used by bell 103 or other modems, but if they aren't as robust that could partially explain the discrepancy you're describing