I wonder what the page size is on his system (and what effective alignment his pointers have). If it's 4K, the sizes look really close to 0x1000 and 0x2000 - maybe crossing page boundaries?
It's because of cache addressing conflicts. If two addresses have the same cache key your cache suddenly doesn't work anymore. And many CPUs just use the low bits instead of hashing it.
L1 caches are usually N-way associative, so that should only become a consistent problem if you access N distinct addresses with the same key (in this case the same offset (with likely 64-byte granularity) relative to a 4K boundary).