'Lumps and bumps' is a very non-specific phrase. In English it is the rhyming sound of the phrase which suggests a meaning just as much as the actual definition. I'd say that it's simply a standard colloquial expression, covering all sorts of irregularities in a surface. You can't really break it down into component words and expect it to make perfect sense.

Regarding your question, there is no difference in the meaning. If used in other contexts, a lump is a piece or mass of indefinite size and shape. Example : The woman found a lump in her breast. A bump is a swelling of tissue. Example: He had a bump on his arm after falling off his bicycle.

No difference. I'm not sure why it would be written that way.