You seem to be an order of magnitude off on your ARPU calculations. $8.15 billion in revenue (p. F-4) / 5.9 million customers (p. 1) = $1,381 ARPU
Either that, or you're mixing up monthly with annual. On p. 4, they calculate their monthly revenue per residential customer relationship at $108.
However, you are generally correct that the cable business is a tough business to be in. Lots of labor costs, lots of capex, lots of interest costs. On an ROA basis, they're not doing that much better than the regulated utilities.