That struck me as an odd choice, too. On average there's no difference in false positives, but the smaller the blocks, the more likely they'll be saturated. Since there are 6 leftover bits in the hash anyway, there's no cost to increase the two 5-bit values to 6 bits and the block size to 64. You'll have a lot fewer hot blocks that way.

With blocks this small there's also no reason not to optimize the number of hash functions (albeit this brings back the specter of saturation). There are no cache misses to worry about; all positions can be checked with a single mask.