The only explanation I could think of is this:
2.34 is the average time spent for this prop value, by 584,965 visits.
When you correlate the prop with "Time Spent per Visit - Bucketed" which is a dimension, you are asking Adobe which values of "Time Spent per Visit - Bucketed" and "Prop8's this value" were seen together, and what was the Average time spent per visit for these values seen together.
The Bucketed values are not "bucketing" the 2.34 of the prop's value, they simply letting you know what the Average Time Spent per Visit (Minutes) was for these Bucketed values and were also seen with the prop.
In other words, I could be in the "1 to 2 hours" value and saw this prop, my Average Time Spent per Visit (Minutes) was 5.72, and for this prop I spent 2.34 minutes average.
In other other words, the metric "Average Time Spent per Visit (Minutes)" for Bucketed values is not dependent on the prop's values.
This should be verifiable by looking at the "Average Time Spent per Visit (Minutes)" for these bucketed values without prop8 and they should be same as this screenshot.