While the modem does check software when it starts, the amount of data used is very minimal. The actual software/firmware updates/upgrades are usually done many months apart. I know you have the HT1100, which I don't have, so I don't know when the last one was done, but I believe the last one for the HT2000W was done about six months ago. And, though I could be wrong about this, when a software/firmware update/upgrade is performed, the data used for such is not deducted from the customer, as the modem is in an update/upgrade state rather than a normal operational state.
If you're seeing the "Software download status" in a state of "Updating" on a regular basis, other than when you first power up the modem, this might be indicative of a problem. I don't want to say that it is for sure, as I'm not 100% positive, but I'm pretty sure you should only be seeing the status of updating at modem startup and during the actual modem software/firmware updates/upgrades, which happens months apart.