802.11a/b/g Performance Chart

It means that there is a beaconing 802.11b access point in the vicinity, with a connected 802.11b client, that is NOT moving traffic. 802.11g will slow down to listen for traffic from the 802.11b device if it hears a beacon. No traffic is necessary to create the slowdown. The degree of slowdown varies with the type of flow control mechanism. Since such flow control is usually off by default, the higher value is used in the table. There are some details in the paragraph under the table on Page 1. If you want the exact explanation, Atheros also published their test methods at: Page 3 is proclaims: "Since the key feature of 802.11g is backward compatibility with 802.11b, throughput tests should be done with an 802.11b client device connected to the access point but otherwise idle. This setup ensures that the 802.11g network is operating in an 802.11b compatible mode."
Other papers are at: methodology is similar to those used by the nifty product performance tests on Tom's Hardware site. For example, here's the WRT54G: 802.11b client connected but not moving any traffic. There are quite a few beacons and managment frames belched by non-active 802.11b access points sufficient to drive a typical 802.11g system to a crawl.

What does the "802.11g (with 802.11b)" mode mean exactly?
Does it mean an 802.11g AP with active 802.11b clients or just the activation of compatibility setting in the AP config, even if there are no active 802.11b clients?
Thanks.

I would think it means with both active. My 802.11g router has "b" enabled, for certain visitors. With just a "g" connected, I get 26.4MBpS on TCP "iperf" testing. Running iperf -d to test both directions, I get 13.1 Mbits/sec plus 14.6 Mbits/sec. I think that's a point that Jeff has raised in the past. 54Mbps is shared on the WAP.
I was able to write to a shared Windows disk drive on the wired systems from the wireless G at about 21Mbps.

Thanks for the explanation.
Since I am using two WRT54GS wireless routers for a WDS bridge with no wireless clients, I am going to set them to G-only mode, just to be on the safe side.
Thanks.