fake car mot

I'm just wondering if anyone has ever had same problem as me . I got a car a few years ago (vw polo) from a used car dealer in my area. when I purchased the car it come with a fresh 12 months mot which had no advisorys on it . so when it comes for me to get the mot done 12 months later it failed on something to do with the exhaust. anyway we got it fixed and through it mot but later when I come e to check the cars mot's from the past on a app on noticed the year I got it had failed on the same thing . so how did the seller get it a new mot with no advisorys to sell it to me .was it a fake ? as when I got it done why would the same thing pop up which surely he should of got fixed to get it to pass the mot when he sold it me . now I'm about to buy a new car which this time I checked it's past mot's on the car and before I paid any cash that the advisory from the last mot would be sorted and he said yes it will come with a fresh 12 month mot with no advisorys but I'm now thinking he hasn't done the jobs and somehow just got it a mot . has this ever happened to anyone else and how do they get away with it ?