Remember seeing a post like this a while back and some dude who makes glass containers commented that this is a manufacturing defect, something about uneven cooling I believe. Too lazy to go find the other post, but I assume someone will dig it up after reading this post.

Yep.

On perfume and Aftershave bottles it is an intentional inclusion. You know it's intentional because the tube is always long enough to bottom it out.

If it was meant to not be sloped, you would end up with some left in the bottom that the diffuser couldn't reach.

Every bottle I have had (both expensive and cheap) had a similar internal design.