perhaps because the WRT54GS is the old and far slower 11g only speced device, and on binary or other compressed files they are going to get around 15Mbit at best over the wireless link , and never more than the 22MBit/s limits 11g can reach, even with data spikes, and perfect conditions, even when useing the lightest WAP security turned on,.
perhaps the old 11g average 15MBit/s is an acceptable wireless speed for most people on the 20Mbit/s or lower package , but if your paying for 50Mbit/s theres no real wish to artificially restrict your max data throughput i assume.
heres the old
detailed WNR2000 wireless N router review
http://www.smallnetbuilder.com/content/view/30615/96/