Forum:Question about the corpse (huge spoilers; don't look if you haven't finished)

Just something that I was on my mind, because I was distracted during the scene...

Big Mama has Big Boss's corpse from what I understand, but then Liquid gets it and burns it and Big Mama in attempt to rescue the corpse dives into the flames... However...


 * Huge Spoilers - Don't Look If You haven't fully beaten the game*

After the credits, it's revealed the corpse burning was actually Solidus and Big Mama hired Raiden to replace the Boss's corpse with Solidus's.

The question I have here is, why did Big Mama think the corpse that was being burned was Boss and dive into the flames?