To add on to Barry's info, you might also want to check your net statistics to confirm that you have the right IP address.
Full instructions written for my mother in mind:https://raidenworks.com/2012/03/10/latency-to-your-broker/
Is it possible that Volume also counts changes in other things? e.g. there's no price change but a 'tick' comes, caused either by some change in
1. contract specifications, e.g. Margin maintenance, Swap long/short. (infrequent)
2. change in margin currency price, in cases where quote currency not in the same denomination as margin currency, thereby affecting the pip value, e.g. indicator attached to EURGBP while the margin currency is in USD, no change to EURGBP price but GBPUSD moves enough to shift the pip value. (I suppose this could be simply tested with the indicator attached to EURUSD, can't do it now as weekend)
3. Or simply it counts repeated 'ticks' of the same price, which we otherwise would not see in the Market Watch?
Or worse, the entire indicators architecture (not specifically your indicator, to be clear) is lagging and misses some price changes? (unlikely?)