frame parameter `buffer-list' - bug? if not, what changes it?

[Drew Adams]

I have Emacs 20.7 on Windows (GNU Emacs 20.7.3 (i386-*-nt5.1.2600) of Thu
Dec 21 2000 on buffy).

I use only one-window frames (pop-up-frames non-nil). Each buffer is
displayed in its own separate frame.

What is the significance of the frame parameter `buffer-list'? The doc says
only this:

  `buffer-list' A list of buffers that have been selected in this frame,
                ordered most-recently-selected first.

Even if a particular frame has only ever displayed a single buffer, this
parameter can apparently have various values. For example, here are two
different values of `buffer-list' for the frame *Help*, which has never
displayed any buffer other than buffer *Help*:

 (buffer-list #<buffer *Backtrace*>)

 (buffer-list #<buffer foobar.el> #<buffer *Help*> #<buffer toss-1>)

What is going on here? Is this a bug? If this is the intended behavior, then
just what is that behavior? What determines (changes) the value of
`buffer-list'? I can find no doc on this.

Motive: I'm trying to test for equality (with `equal') between frame
configurations, and the same frame with all other parameters equal is
showing up with different values for `buffer-list'. I don't want to have to
create and pass a different equality predicate (that would ignore
`buffer-list'); I want to be able to use `equal', if I can.

Thanks,

  Drew