The intent of this library is to implement the unordered containers in the draft standard, so the interface was fixed. But there are still some implementation decisions to make. The priorities are conformance to the standard and portability.
The wikipedia article on hash tables has a good summary of the implementation issues for hash tables in general.
By specifying an interface for accessing the buckets of the container the standard pretty much requires that the hash table uses chained addressing.
It would be conceivable to write a hash table that uses another method. For example, it could use open addressing, and use the lookup chain to act as a bucket but there are a some serious problems with this:
So chained addressing is used.
There are two popular methods for choosing the number of buckets in a hash table. One is to have a prime number of buckets, another is to use a power of 2.
Using a prime number of buckets, and choosing a bucket by using the modulus of the hash function's result will usually give a good result. The downside is that the required modulus operation is fairly expensive.
Using a power of 2 allows for much quicker selection of the bucket to use, but at the expense of loosing the upper bits of the hash value. For some specially designed hash functions it is possible to do this and still get a good result but as the containers can take arbitrary hash functions this can't be relied on.
To avoid this a transformation could be applied to the hash function, for an example see Thomas Wang's article on integer hash functions. Unfortunately, a transformation like Wang's requires knowledge of the number of bits in the hash value, so it isn't portable enough. This leaves more expensive methods, such as Knuth's Multiplicative Method (mentioned in Wang's article). These don't tend to work as well as taking the modulus of a prime, and the extra computation required might negate efficiency advantage of power of 2 hash tables.
So, this implementation uses a prime number for the hash table size.