Distribute spacing set to 0, still not making objects flush.

I have two objects I'd like to abut directly against each other, and so I just discovered the 'distribute spacing' tool. However, even when set to zero, it moves them, but there is still a small amount of space between them. I have checked the objects themselves to make sure that there are no flaws that would prevent the alignment from being perfect. Not sure what I'm missing, but any suggestions are appreciated!

I have also never posted in the forums before - I couldn't find another thread with the same question, so if I somehow did this wrong, please redirect me! Lots of topics explaining how to make objects flush by setting this tool to 0, but nothing about when that won't work. Thanks!