I’m starting to get more and more annoyed with this.
Apparently most people have a hard time to see the difference. Accumulation means that at given points addition (or some other arithmetic function) is done to accumulate new values. Aggregation means Sum of all points in the current context. Thus Accumulation can yield a different result than Aggregation simply because the data set on which the operation is done has changed over time.
You can also see this as a calculated value or a persisted value, if you wish.
The difference between these two when writing business-software can give the customer a faulty summary and thus result in the customer making bad choices. Simply based on the data presented to the user.