Your TRYING to compare Apples and Oranges....this is going to get a bit technical....
Every station can be a bit different in how they "feed" the local OTA stations....and DirecTV/Dishnet. Some provide an uncompressed (up to 40 Mbps, but it might actually only contain up to 19.8 Mbps of actual data) MPEG2 data stream which may or may NOT contain ALL of the sub-channels via the local Fibre-Optic Network. Some provide the SAME (up to 19.8 Mbps with all sub-channels) MPEG2 data stream (usually STATMUXED) that they actually transmit OTA. And I've heard that a FEW odd-ball OTA stations actually derive their broadcast signal from a (different STATMUXED) DirecTV, Dishnet or C/Ku-Band Big Ugly Dish System set up at the remote OTA Transmitter site.
There are a FEW stations that don't carry ANY sub-channels. Per DTV Market Listings found on rabbitears.info for L.A., KCOP Ch9 and KTTV Ch11 are the ONLY stations that don't have a second ATSC sub-channel...BUT each DOES carry a low data rate ATSC-M/H (Mobile/Handheld) sub-channel:
KCOP's TSREADER report is found here:
They have ALLOCATED (an impossible) 79.975 Mbps to their only Video Channel plus 384 kbps for the accompanying AC3 (maybe Stereo or DD5.1) Audio Channel and 96 kbps for the AC3 Audio Channel carrying SAP (in Stereo). However, looking at the ACTUAL Transmission report, at the instantaneous TIME of the SNAPSHOT, 16.32 Mbps is used for Video, 0.39 Mbps for AC3, 0.1 Mbps for SAP and 1.84 Mbps for "Unknown Usage"...which we see (see bottom of yellow block) is for ATSC-M/H (Mobile/Handheld) data stream.
KTTV Ch11 TSReader Report is found here:
However, MOST OTA stations carry MULTIPLE Video programs, each of which is run with either a FIXED or much more likely, VARIABLE data rate allocations using a Statistical Multiplexer (STATMUX).
KCBS TSReader Report shows that it has allocated (an impossible) 79.975 Mbps for the HD Video and (way overkill) 14.975 Mbps for the SD Video channel and an ACTUAL INSTANTANEOUS SNAPSHOT of 14.55 Mbps for HD and 3.05 Mbps for SD channel. Assigning WAY MORE than needed data rates is a dead giveaway that the station is using an UNCONSTRAINED STATMUX. [A CONSTRAINED STATMUX would allocate only a bit more than the max "expected" data rate for a particular channel and hence could be more susceptible to macroblocking.]
KABC, Ch7 also has an UNCONSTRAINED STATMUX (total is well in excess of 19.8 Mbps OTA limit), set up to carry TWO 720p HD (24 Mbps each Allocated) and ONE SD (14 Mbps Allocated) Video channels...although the TSReader Report indicates that at the TIME of the INSTANTANEOUS report the second Video channel MAY or MAY NOT have been carrying a high-rez video program. Actual video data rates were 11.34 Mbps, 5.74 Mbps and 0.93 Mbps.
A STATMUX intelligently looks at the multiple data streams and dynamically allocates the total bit rate to each channel...depending on HOW MUCH each stream actually NEEDS, eliminating "useless" bits so the average data rate is ALWAYS reduced. Hence the HD channel will "steal" data bits away from the SD channel (and vice versa) depending on how the (proprietary) algorithms decide the bits need to go to provide the best video picture qualities and established priorities. It would be very rare for two (and esp. more) channels to need extra bit rates at the same time...but if they do, macroblocking will occur. The trick is to don't try to carry TOO many HD channels at the same time. One HD and a few SD or two 1080i HD and perhaps three 720p HD channels is commonly found in OTA stations. Multi-pass STATMUX's, such as are used on some C-Band Satellite channels (esp. Movie Premium channels) are even more efficient, allowing even lower bit rates for HD cable channels such as HBO, SHO, etc.
So when you're looking at an OTA signal, you're usually looking at a "compressed" data rate that might be "missing" bits during fast moving scenes.
Cable channels have TWICE as much data rate to work with...which means they can easily support two or three HD channels plus several SD channels on the same QAM-256 channel....or as many as 13 SD channels (as I've see on my TWC-San Diego system). It ALSO means that the STATMUX is LESS LIKELY to run out of bits to assign across the Multiple HD and SD channels...there are far more channels to "steal" from when an HD channel experiences lots of motion in the video signal. Also note that those OTA stations that provide the output of their STATMUX are ALREADY possibly degraded vs an UNCOMPRESSED MPEG2 data stream feed via Fibre-Optic network.....so YMMV.....