A Nearly Perfect Copy
Error rate after copying and proofreading is ~ 1 in a billion for most human sequence.
Highly repetitive sequence: aaaaaaa and gtgtgtgtgt etc. is more error prone: aaaaaaa vs aaaaaaa aaaaaaa aaaaaaasecond form is nearly as stable as first..
Transversions (G~A) and (T~C) are more common copying errors than transitions.
More A around than anything: ATP has other uses, and is converted to dATP efficiently.