It's not always just about "saving fuel." There also is some economic relationship between time en route and airframe hours. Recurring maintenance C-checks and D-checks cost millions of dollars.
If, for example, an airplane is flown at M.86 instead of at LRC M.84 (average) for 3600 hours (typical annual utilization), then the difference could be 18,000 miles. That translates into 37 hours of airframe utilization, equivalent to 4 "extra" Atlantic crossings (and additional revenue) between expensive maintenance checks.
Depending upon the cost of fuel, increased utilization by flying "faster" can be more economical!