How do retakes affect your Uni offers?

Hi, i'm an AS level student, and after my C2 exam today and C1 on Monday, its safe to assume my maths grade this year will probably be a D or a C. My other subjects being, Biology,Chemistry and History. If i got AACD and continued with the two A subjects and the D in maths, but planned to retake C1/C2, would universities know this? Im just abit confused as to how any one with crap grades that then retook and came out with good grades would manage to get a place at a good uni for Law or something? Because if Unis don't know youre retaking, won't they give you offfers based on the AAD grades? Do you just have to apply for places with lower entry requirements and lower ranked uni's even though you may come out with AAB or AAA overall? Thanks in advance!