AFAIK a compressor like zip builds a dynamic running table of frequent byte sequences; the resulting archive is written in such a way that when you decompress i

AFAIK a compressor like zip builds a dynamic running table of frequent byte sequences; the resulting archive is written in such a way that when you decompress it, you re-build the table in the process. So if you concatenate files A, B, and C and then compress the result, then by the time the compressor starts compressing the data of C, it will have that table built from A and B. To extract C, you'll need to re-build the same table and thus you'll need first to decompress A and B. In a zip file each entry is compressed individually; this gives random access, but worse compression rate, because the table is not re-used between files.