Different bandwidth of TX/RX

MAX2831 is a 802.11b/g transceiver chip. It's a direct conversion (Rx/Tx) transceiver. In its data sheet we can find that it's RX LPF and TX LPF has different bandwidth. Bandwidth of RXLPF and TXLPF is 8.5MHz and 11MHz respectively in mormal mode. Can anyone tell me why the RX/TX LPF has different bandwidth?

It depend on data rate on Tx /Rx .
Let say TX throughput is high at 11MHz but your RX is noisy and hence at lower data rate 8.5MHz.
these filters are programmable.
You can find the same from the below text copied from page 22/35

Receiver Baseband Lowpass Filter
The receiver integrates lowpass filters that provide an
upper -3dB corner frequency of 8.5MHz (nominal mode)
with 50dB of attenuation at 20MHz, and 45ns of group
delay ripple in the passband (10kHz to 8.5MHz). The
upper -3dB corner frequency is tightly controlled on-chip
and does not require user adjustment. However, provisions
are made to allow fine tuning of the upper -3dB corner
frequency. In addition, coarse frequency tuning
allows the -3dB corner frequency to be set to 7.5MHz
(11b mode), 8.5MHz (11g mode), 15MHz (turbo 1 mode),
and 18MHz (turbo 2 mode) by programming bits D1:D0
in Register 8 (A3:A0 = 1000). See Table 4. The coarse
corner frequency can be fine-tuned approximately ±10%
in 5% steps by programming bits D2:D0 in Register 7
(A3:A0 = 0111). See Table 5 for receiver LPF fine -3dB
corner frequency adjustment.

Yes. I know the TX/RX LPF bandwidht can be tuned. But for a transmitter and a receiver, their data rate must be same when comunication. The transmitter can't be fast than the receiver in communacation. So this can't answer my question.