Answer from Shekar Dattatri, Conservation India:
This is actually not a matter of north Indian tigers vs south Indian tigers but one of habituated tigers vs non-habituated tigers.
In some parks, like Kanha and Bandhavgarh, tigers in the tourism zone have become habituated to riding elephants carrying tourists and to tourists in vehicles. When a female tiger becomes habituated, her cubs too learn not to fear riding elephants or tourists in vehicles. Several generations of tigers in the above-mentioned parks have grown up around tourist vehicles and are therefore habituated to their presence. In Ranthambhore, the other reserve where tigers roam about in broad daylight in close proximity to tourist vehicles, tigers used to be very shy decades ago. But in the 70s, some females were provided with buffaloes (as the natural prey base was low) and thus they and their cubs gradually got habituated to human presence in vehicles. Subsequent generations of tigers then grew up with tourist vehicles around them and thus lost their fear.
However, in most north Indian reserves, tigers continue to shy away from humans even in vehicles, because they have not yet become habituated.
On the other hand, in the tourism zones of Bandipur and Nagarahole in south India, some tigers (as well as leopards) have become quite habituated to tourists in vehicles and are often spotted by the roadsides.