The process that Apple lossless uses to "compress" the file reduces the file size but not not delete any data. Think of it like a ZIP file, which reduces the size of a file but when upzipped the data is exactly the same as you started. Apples lossless and flac do a similar process. There are well known algorithms for doing this type of "compression". They look for patterns that they can store in less space. So the "compression" uses less space but maintains all the original data. When the file is played back, the file is "de-compressed" to its orignal form. The earlier discussion in this thread is about possible subtle differences in Apple lossless encoding, but that is a different issue that the 50% difference in file size.
My copy of Pop Pop arrives today and I will doing some listening this weekend.
My copy of Pop Pop arrives today and I will doing some listening this weekend.