This margin between testers means that the standard can vary. My advise is firstly be wary of garages offering discounted test fees, i have seen cases were the garage offer cheap tests but find faults like incorrect alignment of headlamps this is an easy way for them to say your headlamp aim was high but we have adjusted it and you car has passed, of course they charge for the adjustment thus generating more money to offset the cheap mot.
This is not to say that if you pay fully price for the mot that you will get a good job..... no no.
However there is one way to ensure you get a fair test. All councils must provide an MOT testing facility to the public they probably wont advertise it but it will be there if you ask, and here is the important bit, they are not allowed to carry out any remedial work so there is no incentive to 'create work' i have found that councils also seem to have the best, newest equipment which is maintained to a very high standard.
The only down sides to using the council test station is that if your car does fail you then have to find a garage to carry out the work but you are then in a position to tell the garage exactly what work needs doing and if they don't do it correctly you will find out when you go back for a retest and I think it would be a very stupid garage that would argue with a council test station. other down side is that the mot test wont be discounted.
Last of all take 10 minutes a week before your test to check your car your self, check all your :- lights, horn wipers, washers seatbelts, locks and tyres. if you do this a week before the test you will have time to fix or get them fixed before the test, halfords will change bulbs and wipers and if it saves your car from failing and having to pay a retest fee it will be money well spent.