Who is your ISP and how much per month are you paying.
Reason I ask is I bet if you are using C-NET or some freeware program that test isn't 100% accurate.
Several reasons
Cable (Comcast etc) is based on fraction service, you can tap out the full speed but not sustain that. The main central neighboorhood hub could have a long metric and many hops.
DSL is better but still not perfect, you can sustain 500-768kbps and nice bursts on 1.5mbps+
T1 you can have fractional or fully functional, either way you'll see a continuos 56k per pipline with up to 64 lines.
And yes all routers will lengthen the metric in which packets are sent and received to each
PC. Not all routers especially consumer level ones feature Full-Duplex switching.
~Bob