Darkened image on using mixed PDF while lights PDF and cosine PDF images look similiar to one in book.

[mTvare6]

I've been working on implementing the code in Rust and noticed that using a simple mixed PDF approach (0.5 * pdf1 + 0.5 * pdf2) results in an image that's much darker than the one in the book. I thought this might be due to the fact that lights_pdf is much greater than 1, while cosine_pdf is less than 1. However, statistically, the effect should cancel out.

To test this, I switched from using MixedPDF to random_double(), storing whether the value came from the light or cosine PDF, and then using the appropriate value function. This approach produced an image closer to the reference, but the front face of the left box still appears darker. I'm not sure what could be causing this.
Mine:

Given(Image 11):

Storing if first or second pdf approach.