See bug report and some suggestions how to improve it here. Activity: workout line is off for distance-based steps
TLDR, intervals.icu simply converts distance to time in that chart based on what it expects the pace to be. This is fine when it’s still in the future.
But with an completed workout it could (as demonstrated by Garmin) switch to a different algorithm and make the blue line perfect. The algo is not complex and the data is available, just not the priority/dev time. 
vote that bug report if you think it’s important.