Maybe that's down to Senna tending to dominate races from the front and hence taking it easy towards the end of a race - and his era exactly coincided with the no mid-race refuelling rules. So Senna was deliberately going his slowest at the stage of the race that the cars were at their fastest.
This was true during the San Marino GP of 1988
Prost started 2nd, but fell behind a few cars after a duff start, and had to navigate his way back to 2nd place. By the time Prost worked his way back up to 2nd, Senna had built up a 8 second lead, which he managed until the end.
Prost pushed like a madman at the end of the race, which had the McLaren team worried that he was going to run out of fuel. Senna deliberately slowed on the final lap to ensure that his car would make it (despite no indication that he actually would run out of fuel, the concern according to ESPN was that Prost was the one under threat). It was one of the most carefully managed and expertly controlled races I have observed.
In contrast, Senna could push towards the end of races if he wanted to. At Germany 1989, he fell behind Prost due to a bad pit-stop (his 2 second lead, became a 4 second deficit solely based on the pit-stop). Senna slowly closed the gap on Prost throughout the final stint and got within a second. Then Prost's gearbox expired, and Senna took the lead back (a nice luck adjuster).
Senna did make errors due to pushing when it was not needed (Monaco 1988), but the majority of the time he was excellent at managing his pace.