T1 speed on different OS’s

Just curious! We just got a T1 installed in our office. When I do a speed test from
computingcentral.msn.com — our two Linux boxes (one on a pentium pro 200, the other is a low end celeron), clock in at
~1300 mbps.
Our NT4.0 workstations clock in at half the speed ~600mbps! (boxes are low end celerons).
All boxes are on a peer to peer network, on the same segment.
Is this normal? Are there some things on NT setup I should be looking at??

Thanks
Curious