Hops are often blended and combined by those who brew at home and by commercial producers as well. There are some obvious reasons for mixing brew hops. the simplest reason is that you can get blended flavours and aromas that are distinct from a single flavor from a single hop variety. Using mutiple types of brew hops also makes it easier to produce a more consistent flavor from recipes as well. There are different levels of acids and oils in each variety of hop. However, the levels of any single hop  are also subject to change every growing season. Moreover, the ptoperties from hops from any two plants from the same season may differ consderably. Thus, you can minimize the differences that may occur in any one hop by blending two or more varieties together. Using different hops allows you to experiment with the cooking process as well. You can add different amounts of hops (though no more than the total!) in the boiling process or with dry hopping to produce different flavours. Lastly, the varieties of hops have different price points. One way of reducing costs is mixing a cheaper variety with a more expensive one.