Volume in the Charts is different than what gets returned from the API's
It's not clear which API call that you are using so I'll give my two ideas on what the problem might be:


If you are using the "getmarketsummary" API call and calling that
every minute then you will get wrong data because this is based on
24-hours. In my opinion, this is a useless feature.
If you are getting the market trades and then calculating the OHLCV
value in 1-minute periods, and those values don't match up with the
Bittrex candle charts then you are probably seeing the 2 to 4 minute
delay that Bittrex has. I use a service (ClueDex) that removes
that delay and I can literally get 1-minute OHLCV candle data as
soon as the minute closes.