Sometimes it isn't. In a tax office we use spreadsheets to track the changes in each investment account, for each month to get an accurate ACB for the year end at December 31. You should end up with the same average cost that they show. Sometimes they miss things.
You might have return of capital, you have reinvested distributions, which also increase the number of units at the cost that was recognized on the date of distribution. Sometimes there are dispositions to pay fees, that also reduce the number of units, but don't necessarily change the cost of the units.
Sometimes it isn't. In a tax office we use spreadsheets to track the changes in each investment account, for each month to get an accurate ACB for the year end at December 31. You should end up with the same average cost that they show. Sometimes they miss things.
You might have return of capital, you have reinvested distributions, which also increase the number of units at the cost that was recognized on the date of distribution. Sometimes there are dispositions to pay fees, that also reduce the number of units, but don't necessarily change the cost of the units.