bandwidth of the optical modules

I ahve a question on the bandwidth of the optical modules. We know from the communnication theory, the chip set (such as TIA and LA ) in optical modules will always set thier bandwidth at 0.6~1.0 data rate to trade off the noise and ISI. But I found some products's bandwidth is largerthan the date rate, e.g., for the 155MHz data rate, the bandwidth set 250Mhz in typical case. Can anyone help to clarify it? Is any other considerations on it?

Thanks