but their number won't be accurate is they don't measure ALL data, only specific types of data.
What do you mean by that? I have a Asus router and it is accurate.
Some routers have automatic speed tests they run periodically, but the router doesn't add that to the usage figures, and running speed tests let's say on a daily basis, can mean GIGS of data used, but not accounted for. Some people have their routers set to run them hourly, I cann't even begin to tell you how much data that uses!
Some routers are setup to perform DNS, and while they measure network traffic from each device, dns lookup information isn't counted because the router itself is doing the lookups. There are also other types of data that some routers don't monitor properly, such as crc or checksum data.
