So now you know. Though that perhaps wasn't exactly what you were asking. Poor preparation; nerves; over-confidence; rushing; one-off "odd" situations—maybe these are closer to what you meant. But the good news is that, in the last decade, (1) the average pass-rate has risen 4% ; and (2) over FOUR TIMES as many people pass with ZERO faults !
South Birmingham rates DO seem to be below the national average, but that doesn't imply examiners are behaving unfairly or test routes are more challenging (though they could be).
The national average is currently 47% from the 1.54 Million tests conducted 2015-16 (+3% for men; -3% for women). See more here.