You may have had other issues with your existing cabling. Certain miswirings will allow for slower mbps speeds.
It is true that Cat5 cable is capable of supporting 1000mbps in certain circumstances. I have this in my own home network. When I first began wiring my home 10 years ago, Cat5 was the standard. As I have upgraded Macs and Switches, my connections have automatically taken advantage of Gigabit speed.
I'm expecting that with my short cable runs it may be possible to reach 10gbps once I have a Mac and Switch capable of those speeds.
The thing is, the number of conductors and the fact that they are arranged in twisted pairs hasn't changed since Cat5 at least. So the speed is something the networking hardware negotiates upon establishing link using only the electrical characteristics the networking hardware can quantify. If both ends determine they can send and receive a certain speed over a given connection, they will settle on that speed regardless of the cable's rating.
It is even possible that a wiring fault or poor quality installation could reduce a Cat6 cable to 100mbps or 1000mbps on a 10gbps hardware network.
What's weird is I had a NetGear 1Gb Hub with cable sense. I had the 5 cables connected that my bro did for me and the Hub was negotiating them at the 100 BaseT (I didn't know at the time), the lights were saying they were 100 BaseT. Then I went and had to buy 1 additional cable and I plugged it in and the light changed, at the time I didn't even know the Hub had different color lighting to sense the cable bandwidth. I said why is this a different color from the other ports. Then I said Mother Effer, those cables my bro got weren't Cat6, and the new one I got was Cat6, and the Hub lights were sensing the difference. So I ordered from Monoprice all new Cat6 cable, and all the lights lit up signifying 1Gb. The only thing I can think of is my Brother must have gotten me Cat4... No he got me Cat5, I think you are thinking Cat5e which supports 1Gb...
Here is a chart I found: