At least 2 people replied to you referencing the lossyness of the encoder, but that is actually completely irrelevant and unnecessary to answer your question, a

At least 2 people replied to you referencing the lossyness of the encoder, but that is actually completely irrelevant and unnecessary to answer your question, as it applies perfectly well to lossless compressors too. See for example Zopfli and ZIP, which is obviously not lossy. The situation exists because there are many alternative ways to represent a given, fixed output file, so that the job of the encoder is to seek through this search space and try to find a size-optimal solution, or a good approximation thereof.