Is there a reasoning behind this?
Yes, there is. Most stock exchanges in the world open either at :00 or :30 of an hour, while the India stock exchanges are the only ones that start at :15 of an hour. Most professional charting softwares were made for the International exchanges and not for the Indian exchanges in particular, hence they are all made to take :00 as the starting minute point. In the case of NSE and BSE, the starting minute is :15 of an hour as the market opens at 9:15 am.
If you use the approach of starting at 9:00am, the first candle is a 45 min candle (since market opens at 9.15am and there’s no chart data from 9am to 9.15am). If you start it at 9.15am, like in Kite, the last candle of the day which is 3.15pm onwards will be a 15 min candle and not a 1 hour candle.
I’d say, you rather have the first candle of the day as a proper 1 hour candle, since you might have to track the market for the rest of day based on the first hourly candle formed. If the 1st candle itself is a 45 min candle, the rest of the day’s candle data might appear skewed. Also if the candle were to start at 9:00am the first candle of the day would be 45 mins and the last candle would be 30 mins. So here there are two candles shorter than 1 hour. In case of starting at 9:15 AM, only the last candle of the day would be shorter than 1 hour.
But this is debatable. 
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