How does Coinbase / GDAX calculate volume in historical candles?
I can't answer for the discrepancy that you're seeing, but the "right" way to calculate volume would be to add the "total" column from each trade that occurred during the period. That is, assuming that candle charts are based on stock market candle charts.

It doesn't surprise me that you're seeing this discrepancy, however. I've noticed a few different cryptocurrency APIs that have no idea how to calculate OHLCV data the right way. If your candle data isn't correct, then none of the traditional TA-Lib functions will mean anything!