Broadband daily usage value not matching what router is reporting

[#1]

Had a look at "View My Broadband Usage" and was surprised how much data was transferred on Tuesday 28th.
Decided to check router "Helpdesk" tab that lists sent/received totals for the multiple days device has been running, and it is less than the total used on the above day according to "My Account" webpage.
Any advice to explain reason for the discrepancy?  And which should I believe?

[#2]

What's the Uptime showing on the Helpdesk Tab? You also need to add both the sent and received traffic together as they both count towards usage.

[#3]

Uptime was 3 days and around 2 hours when I checked router.  And combining the sent and received totals was still less than reported usage on a single day on website accounts screen.

[#4]

I know with the TG582n, the data sent & received gets reset on the router if there's a drop in PPP session (not sync but that will also result in PPP drop - usually) I'm not sure at the moment if that's the case on the 2704n, I may be able to confirm later.

[#5]

If you drop the PPP session by clicking Disconnect on the Home page in the Internet Status box it will reset the Data sent/received figures. Don't forget to click Connect to start a new PPP session. Note this is not disconnecting sync if you are using ADSL, however a loss of sync will cause a loss of PPP session. Also note you can lose PPP session on occasion due to maintenance work, either on the BT backbone network or the relevant part of Plusnet's network. See http://usertools.plus.net/status/archive/1438435750.htm and http://usertools.plus.net/status/archive/1438624803.htm for example.