Fasting blood sugar 7.5 but Hba1c 5.6

FBS is a snap shot of your blood sugars at one particular moment in time. HbA1c is an average over 2 to three months.
If your BG is 7.5 fasting but the rest of the day is a lot lower then the average over the three months will also be lower.

That is one likely answer to your question.

Additionally, A1c is measured as a % or mmol/mol, where as your BS is measured as mmol/l, so an A1c of 5.6% (which equals 42 mmol/mol) actually equates to an average BS of around 6.3mmol/l.