question about 'divisible by some large powers of two' in 'Cache Associativity' section #228

in 'Cache Associativity', the author says:

When the array size is a multiple of a large power of two, then the indices of the “hottest” elements, the ones we likely request on the first dozen or so iterations, will also be divisible by some large powers of two and map to the same cache line — kicking each other out and causing a ~20% performance decrease.

I think only index subbits are related whether memory should be stored to the same set, while this doesn't imply one divisibility relation