It's like Nokia do every little bit of the market and the higher the number the slightly higher the spec unlike iPhone that just makes two versions and expect everyone to pay the price.
Shame they don't make a Nokia 305 to upgrade the 301 was a great phone but poor battery and no volume keys. The 515 is great but could argue is advanced. I want a simple music phone with volume keys. and there are some great Nokia's out there. S40 still lives on. It's a shame the golden age of mobile was 10 years ago but as the competitors get less and less it will gradually get more boring