from my memory, both ARJ and LZH outperformed ZIP for my benchmarks. I used to stripe archives across as many floppy disks as I had until I eventually got a CD writer...
According to http://fileformats.archiveteam.org/wiki/ARJ , ARJ uses something similar to LZH's lh6 algorithm, which is LZ with a 32k window + Huffman, so the difference could be due to ARJ's reduced file format overhead compared to ZIP, or perhaps slightly better Huffman codes.