[boost] [circular buffer] logic behind set_capacity vs rset_capacity?

I was using Boost.Circular_buffer for the first time today, and noticed that the set_capacity and rset_capacity methods did the opposite of what I expect; namely, set_capacity will clip elements from the back and rset_capacity from the front, despite the fact that insertion using push_back will overwrite elements from the front. Is there any rationale for this operation?