I am trying to book a one way flight from Frankfurt to Delhi. While checking the Vistara official website I noticed that on two separate date 02.10 and 16.11. the flight is taking 08 H 40 m and 09 H 50 m resp. Both flights are taking same aircraft Boeing 787-9. Has the website confused because of daylight saving. Here is the screenshot of the booking for both dates:
It's most likely operational.
The change coincides with a daylight savings time change in Germany on Oct 29. For any DST change the schedule needs to be adjusted. They either have to move the departure one hour forward or the arrival an hour backwards. Which one they choose depends on a lot of factors: connections, crew availability, maintenance intervals, etc. Another big one is availability and cost of departure and landing slots at the airport.
In this case I'm guessing Vistara looked at the schedule, decided to keep their departure slot in Frankfurt (which is a busy and expensive airport) and adjust in Delhi. They may have decided to go extra slow either because they couldn't get a better landing slot or for fuel optimization (or some other operational reason).
The other major airlines on this route are Lufthansa and Air India which both only take 7:35-7:45 so after the DST change Vistara is more than 2 hours slower than the competition. If someone is interested: here is a table how all three airlines deal with DST
I don't think this is a glitch, as least not in the sense "website does not show correct times or duration because of DST change".
When time changes at either end of the flight, airlines have to adjust their timetables, and with additional considerations such as night curfews (not applicable here, but could be applicable to other flights, which would have a knock-on effect on aircraft or crew planning and availability) or connections to other flights, it often happens that they make significant changes to their whole schedule (not just that flight). They may also take that opportunity to add or remove destinations or flights.
So they will end up basically re-planning their whole schedule.
Now there could be several reasons they decided to plan more time for that flight:
They noticed they were often late (not saying this is the case here, I haven't looked at the history), so decided to add some more buffer (remember, this is a scheduled time, the flight may well arrive much earlier than the schedule). For flights from Europe, this is especially important due to EC261 compensation they may have to pay out for delays, particularly if there are onward connections.
They couldn't get matching departure and landing slots at the two airports, so they had to adjust one.
Seasonal weather patterns may affect winds and/or routing.
They had to change their usual routing due to geopolitical considerations (wars) but hadn't updated their schedule until now.
They want to save fuel (not convinced at all this actually works out -- probably a good question for aviation.SE).
There are probably quite a few other reasons that haven't come to mind.