The difference is there is no criteria for an air bag fail in the mot only the srs lamp, and if any mot tester can show you the item in the mot test that an airbag light that is on fails an mot then he needs to read the mot manual. I have passed many with the lamp on and have even taken it up with vosa and they agree only srs light fails hope this helps

That makes absolutely no sense... Please read the manual tells us...
Control indicator v for belt tensioners The belt tensioners are monitored electronically together with the airbag systems. Their operational status is indicated by control indicator v in the instrument panel. When the ignition is switched on, the control indicator lights for approx.4seconds. If it does not light, does not go out after 4 seconds or lights while driving, there is a fault in the belt tensioner system or the airbag systems; see page89. There is a possibility that the belt tensioners or the airbag systems will not be triggered in the event of an accident.Have the cause of the fault remedied. We recommend that you consult your Vauxhall Authorised Repairer.
From page 81 of the owners manual...