You get RAW marks by correctly answering the questions
Those RAW marks are converted into UNIFORM marks (UMS) according to a scale they work out after the exams - the scale is adjusted based on the expert analysis of the examiners to compensate for more/less difficult exam papers.
You will be told your UMS score as well as your unit grade. A maximum 400 UMS are available for the 4 papers combined. You need a total UMS score of 320 from the 4 units to get an A. You said you had only taken 1 unit? Therefore there are still 300 UMS available from the other 3 units - if you had a B from the first unit (at least 70) that means you could get up to 370 - well above the requirement for an A overall.