to my current understanding Universal Hashing is a method whereby the hash function is chosen randomly at runtime in order to guarantee reasonable performance for any kind of input. 

I understand we may do this e.g. in order to prevent manipulation by somebody choosing malicious input deliberately (a possibility of a deterministic hash function is know).

My Question is the following: Is it not true, that we still need to guarantee that a key will be mapped to the same address every time we hash it ? For instance if we want to retrieve information, but the hash function is chosen at random, how do we guarantee we can get back at our data ?