Time Interpolation Checking

[3vivekb]

Sometimes the interpolated time is too fast and kinda changes.

Look at the 181 in this instance:
https://tripplanner.vta.org/planner?from=380%20N%201st%20St%2C%20San%20Jose%2C%20CA%2095112%2C%20USA&to=42633%20Shoreham%20Park%20Ct%2C%20Fremont%2C%20CA%2094538%2C%20USA&modes=TRANSIT%2CWALK&days=M-F&arriveBy=false&date=12%3A21%3A2016&hour=14&minute=0&fast=false&safe=true&flat=true

Sometimes the trip takes 13 minutes, and sometimes 26 minutes.
Why?

Neither of these points are timepoints so OTP is doing some interpolation.
http://www.vta.org/routes/rt181
It doesn't look like times change much. Perhaps there is a problem in the GTFS for this route.