The data plan counter doesn't measure some types of traffic (non-IP packets for example) and I suspect that the widget just reports the same number in a more convenient way. What the carrier chooses to count may be somewhat different, but it doesn't sound like a plausible justification for the huge discrepancy that you see.

I am currently using a prepaid SIM and only connect to Wifi. The data plan counters always show zero bytes as you would expect but running ifconfig in an xterm shows a nonzero byte count for the phonet0 interface even though it hasn't ever been given an IP address.

I haven't selected a carrier data connection or sent or received any text messages since rebooting yesterday, but the phonet0 interface shows:

RX bytes:90416 (88.2 KiB) TX bytes:3616 (3.5 Kib)

In the last few minutes, 16 new packets totaling 192 bytes have arrived.

All that I can suggest is watching the raw byte counts on the interface and see how that aligns with the values reported by the data plan monitor. It restarts from zero after every reboot so note the total before rebooting.