Interleaved assignment is more efficient than block assignment in terms of memory bandwidth. By interleaving, whole chunks of the array can be prefetched at a time and each program instance has their first element to execute. While for block assignment, one instance might try to prefetch their entire block, however another program instance will try to also prefetch their block which can caused complications if one instance evicts another's prefetch.

To sum up @Claire's comment, interleaved assignment provides better cache locality.