I was curious about the discrepancy, and Open Secrets doesn't really report its methodology well here. I did some digging and found that it's the amount of money that is spent on lobbying federal officials, not a number inclusive of all kinds of political expenditures.

It does seem a bit motivated to choose federal lobbying as the initial representation of spending around the gun issue, when it's clearly dwarfed by initiative campaign expenditures, donations to individual candidates, and electoral campaign expenditures.