bandwidth tester speeds

I’m upgrading my network to gigabit and ran the bandwidth tester to see what what speeds I had at 100 Mbs full duplex. I was seeing 30 Mbs, then changed one NIC to 10 Mbs full duplex and the speed went to 41Mbs.
I changed the NIC back to 100 Mbs and got the same 41 Mbs reading.
Is there a limit to the testing speed in the program?

i think only limitation is CPU able to produce packets. and cabling quality.

also do not run the test on the router, you must test through the router:

[Btest runs here] ~~~~~ [Router1]-----[Router2]~~~~~[Test to here]

we are able to run 97Mbps TCP (!) throughput test between two routers P3/500MHz connected thru cascaded 30 switches on 100Mbit Fast Ethernet. Fullduplex gives us around 55-60Mbps in this configuration.