Something better is improved — like when a more advanced computer comes out, or you feel better after a cold.
When we talk about things being better, we're comparing in a favorable way. A great movie is better than a good or awful movie. With practice, you will get better at a sport or a subject like math. Doctors help patients feel better, and mechanics make a car run better. If 51% or more of the people voted for you, then you had the support of better than half the people. When things are getting better, they're improving.