reply to vientito1
Re: what causes difference in usage total? depends on your software that's measuring it. Most likely it doesn't actually count bits, it estimates them based on throughput speeds.
For instance, PFsense RRD graphs take a sample of the speed at certain times. my numbers may be off, but it gives you the concept. For the last day it checks it once every minute, takes the 2 points and gives you an average speed over that time which can then be used to approximate usage. But as time goes on (my router is 3 years old so that would be a lot of data), it drops some of it and goes to a 5 minute average. Well a lot can happen in those 5 minutes. The result is that the "max" throughput changes and the averages decrease. If the through put is a function of those, it would then go down aswell.
Other things to consider are at what point you're measuring.
Lan traffic? - would have inter lan traffic included
lan-wan traffic? would just include requests and no over head from the router
Wan traffic - should be pretty close.
Basically it comes down to how it calculates or estimates the throughput. almost all usage counters are estimates as actually counting them is highly inefficient.