Does AT&T factor up data usage?

Your My ATT bill usage show less than your phone because your phone is an independant and immediate meter of your usage.  But it take from 3-72 hours for calls, text and data to be reported and posted on your My ATT account.    (That's on your bill, see below).  

I am also on a "grandfathered " plan.  It shared, but cheaper than the current plan.  I see no irregularities in data use.  We actually have some job changes, so my data use has dropped a lot in the past few months.  

The data on my devices is very close, not exact, but very close to the independant meter on my phone.

Have you ever tracked it?   Make sure your phone is reset at the end of a bill period, then watch data use all month.   At the end of a bill cycle ATT tries to include all data used and your total on the phone in settings should be very close to ATT bill totals.

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@KeenWood wrote:
Just as is described in many of the posts, it would appear data is not a one-to-one charge...i.e. data shown as being used on your device is not equal to, and appears to be lower than, what is shown on the AT&T data usage indicator. Since data is not "regulated", and AT&T network space I would believe is a premium, "managing" users data rates to "fit" the network makes all the since from AT&T'S standpoint...can someone truly explain why it appears data usage rates are not always equal?? I do not have the mobile share plan and it seems AT&T wants to force users in that direction...which I believe would help them to better manage the network by limiting users' bandwidth more easily once users are "driven" to their data cap.

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Not "always equal" to what?

AT&T charges specifc costs for blocks of data that "you" choose the size of. If you go over, the rate is a different but also fixed price per block.

I track my data on my phone monthly, and it's always within a "few percent" of the numbers that AT&T has on their site. I'm billed exactly what then tell me they will "charge" me. 

If you're not on a Mobile Share plan and you are out of a contract, then you "likely" want to be (you're paying the same rate but not getting "free" phones any more)..  Especialy if you have several users, Mobile Share is much more affordable.

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@KeenWood wrote:
Thank you for your reply...this statement "your provider may account account for data usage differently" means nothing, I guess.

It's hard to reply to an out of context quote (that's likely incorrect).

I have a wireless broadband device, live in a very rural area (no one close enough to connect to my signal), and have literally watched my device rack up data without any device connected.

Your device counter is going up?  (Not the web, the actual device?)
That wouldn't have anything to do with AT&T's end.

AT&T, after working with Technical support, helped us replace the device. The new one worked well for several days...we are now experiencing extraordinary data usage again, yet our usage habits have not changed (don't stream or download movies or video...or upload photos or videos). Phones appear to be close to accurate when tracking usage. The mobile broadband seems to be a different story.

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Is there a laptop or tablet connected?  What else is connected?

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@KeenWood wrote:
Thank you for your reply...this statement "your provider may account account for data usage differently" means nothing, I guess. I have a wireless broadband device, live in a very rural area (no one close enough to connect to my signal), and have literally watched my device rack up data without any device connected. AT&T, after working with Technical support, helped us replace the device. The new one worked well for several days...we are now experiencing extraordinary data usage again, yet our usage habits have not changed (don't stream or download movies or video...or upload photos or videos). Phones appear to be close to accurate when tracking usage. The mobile broadband seems to be a different story.

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The problem might not the device, you aren't accounting for the delay in data reporting and the higher volume of data possible.  Data used on the device may take 3-72 hours after use to post on your ATT usag reports.  

Your mifi should have an independant meter of data use.  Is that meter working?

how close are the numbers?