Basketball Reference identified 24 NBA Conference Finals game 7's, that date as far back as 1971. While betting totals data from 47 years ago (or even a 15 years ago, for that matter) has proven elusive, we a can say that the average combined game 7 score is approximately 191 points and the median is 184.
As is illustrated on the chart below, per besttickets.com, the League-wide points per game average is down materially over about the last 40 years. For instance, in the mid-80s, scoring peaked just shy of 111 PPG, but sits at around 106 today. Further, NBA action in the 1960s apparently featured even more prolific offenses, thrilling fans with average scores of 115 PPG.
Given the much lower average game 7 score relative to average scores in general, even without adequate data to quantify the O/U record, we think we can safely conclude that the UNDER has feasted on Conference Finals game 7's.
This inference fits with the assertion by The Action Network's John Ewing that 1) the familiarity that stems from facing the same opponent seven times in short order breeds solid defensive adjustments and 2) participating in protracted series exacerbates player fatigue. Intuitively, both of these factors would be expected to depress scoring. Ewing offers the impressive cover rate for the UNDER in NBA playoff games 6's and 7's since 2005 as evidence of his hypothesis.
More granularly, we submit that both the Rockets and Warriors are naturally disposed toward the UNDER. Such a statement seems counter-intuitive for the two best offenses in the NBA, however, consider the following statistics:
Houston's and Golden State's combined O/U is 83-110-3 (43%) this year,
These teams' aggregated O/U playing on one night's rest is 50-65 (43%),
The Rockets and Warriors combined OVER is 59-76 (44%) versus the West,
At 5-1, the UNDER has been dominant in this series,
The O/U went 1-4 in the 2015 Western Conference Finals match-up between these same teams
Thus, a study of broad history as well as these teams individual and collective tendencies tilt us toward the UNDER today.
Happy betting!!