The agreements I've seen usually pertain to the area of interest for the company. If you work for a car company, any car related inventions you come up with probably have something to do with working on cars, so it makes sense that your ideas in the area of cars are due to your employment there. If you came up with an idea for a line of children's books, it's plausible that invention was independent of your automotive work.

For a general R&D firm, then the focus of the company is probably harder to pin down, so the agreement probably has to be broader. An employer wouldn't last long if they paid for their competition to get exposure to the industry and to reap the rewards of that work.