Why do trailing separators change the behavior of relative path conversions in Boost.Filesystem?

Given the following code,

namespace fs = boost::filesystem;
fs::path parentPath("/home/user1/");
fs::path childPath("/home/user1/Downloads/Books");
std::cout << childPath.lexically_relative(parentPath) << '\n';

I expect the output to be "Downloads/Books", but instead it is "../Downloads/Books". If I change the parentPath to "/home/user1" (removing the trailing separator), I do get my expected output.

In my real code, the paths often times come in as strings from data. In my context, a trailing slash at the end is unintentional and should be ignored for the purposes of any logic. Basically, in my program "/home/user1" is identical to "/home/user1/". However the logic of boost::filesystem::path doesn't reflect this assertion.

So my question is two fold:

1. Why does the trailing separator affect the behavior? I just can't quite understand why it steps up a directory with ".." in this case.

2. How can I make path ignore or normalize trailing slashes globally in my program? For example, "sanitizing" my path strings before I construct path objects would be too much of a maintenance burden, so I need something more inherent if possible.

Here's a live sample I've been using for testing.