This is not explained in the film
And as far as I've found has never been addressed by the filmmakers either.
I believe the only possible in-universe explanation is that dreams have some consistent properties that are different than reality, that would, for example, cause the top to always keep spinning in the universe of a dream.
This isn't impossible--maybe dreamers consistently think of a top as spinning for a long time, so in their dream they don't ever expect it to fall over. You could argue that, just as Arthur's loaded die totem is treated as a regular die in a dream, a top would be treated as a spinning top that doesn't fall over since that's our expectation for tops.
This would also explain how things that are not possible in reality can be implemented by architects (e.g. the Escher staircases and Cobb and Ariadne being able to walk onto the road after twisting it at a 90-degree angle); the dreamer understands things like gravity to work a certain way such as by anchoring you to the road, whether or not it is consistent with reality.
However, the movie does not specifically demonstrate this to be the case; everything else in the movie apart from tricks that come from playing with the dream universe appears to work as reality would, including physics. So arguably, a dreamer should expect Cobb's top to work like a real-world top, and fall over eventually.
My personal opinion is that, whether or not Christopher Nolan recognized the issue with the top, it just works well for viewers since most people have probably experienced something similar to the top always spinning in their own dreams. Especially, I feel it works significantly better for Cobb's inception of Mal. Seeing the spinning top get locked into the safe in her mind is stronger for the viewers to see versus Cobb leaving a loaded die in there or something like that. So the top's use as a symbol outweighs its actual strength as a totem.