Max bit rates

The estimated throughput calculation done in wpa_supplicant/scan.c is
based on a step function that maps SNR to a bitrate [0]. As I
understand it, these bitrates come from something like mcsindex.com
that maps an MCS index to the corresponding maximum bitrate. My
question is this: where do the SNR to MCS index mappings come from,
and why do they differ from another table I have found [1]?

[0] https://w1.fi/cgit/hostap/commit/wpa_supplicant/scan.c?id=a1b790eb9d75
[1] https://d2cpnw0u24fjm4.cloudfront.net/wp-content/uploads/80211-Data-rates-and-snr-requriements-screen-shot.png