CASINO ROYALE is best explained as a retcon (see
retcon) or a reboot (see
reboot). Either way, it presents a break with the previous continuity and somewhat rewrites Bond history.
Many things happen to contradict the previous continuity:
-Bond gains his Double-O license in a clearly modern time period (there are references to the Cold War being in the past, as well as brief references to 9/11 and the Gulf War)
-Bond meets Felix Leiter for the first time in CASINO ROYALE (whereas DR. NO claimed to be his first encounter with Leiter)
-Bond gains the Aston Martin DB5 for the first time in CASINO ROYALE (whereas Bond previously gained the Aston Martin DB5 in GOLDFINGER)
That said, many have suggested (myself included) that there is no real, honest continuity in the Bond films, and that any attempts to prove there is is nothing but fanwankery. Any attempts at establishing a Bond "history" is tenuous at best - there's little consistency in the franchise.