One possible explanation is that FSRS tends to underestimate the value of decay (Decay tends to be lower than true value when the retention range is narrow by L-M-Sherlock · Pull Request #1 · open-spaced-repetition/fsrs-trained-on-simulated-data · GitHub). So, if you ask FSRS to make a prediction in the R range which is not present in the actual data, the prediction can be quite wrong.
In other words, if most of your reviews are in the 90% R range, FSRS will give a unreasonably high value of interval at DR=70%.