A 70/30 bevel is most often found on Japanese kitchen knives. Knife makers create this type of bevel to aid in steering and control when cutting and will produce cleaner cuts. This is especially important when dealing with exacting techniques, such as breaking down fish for sushi. You have one bevel that is very steep and is able to slice through fragile products without crushing. The other side, with a wider bevel, allows the product to fall off the knife easily without damaging it. In the image below, you can see the general difference between the typical 50/50 bevel where both bevels are equal in size, compared to the 70/30 bevel where one bevel is significantly larger than the other.

To create or maintain a 70/30 bevel is fairly straight forward. Most people would assume that the two bevels are at different angles. This isn't  completely correct. While some blades do have asymmetrical grinds that result in an angle difference of 70/30, a true 70/30 bevel simply means that 70% of the strokes are done on one side vs 30% on the other. So for every 10 strokes, 7 will be done on the larger bevel while 3 are on the smaller one. The bevels will be at the same angles, you're just removing more metal from one side.