Can someone help me to figure out the purpose of the two different filters?

I have this long cable which is carrying two differential voltage 
signals (around 300kHz maximum frequency) through a very "noisy" 
environment. I need to acquire this signal through a differential op amp 
(PGA204) and I was thinking about putting a filter at the input of the 
instrumentation OP AMP(to know more about operational amplifier: 
http://www.apogeeweb.net/article/60.html).

I found this nice filter:
See attachment iGS1m.jpg

But I cannot deeply understand the purpose of the two different filters 
(DM and CM). From what I know, a common-mode noise is a noise present in 
both lines with the same "polarity", while a differential-mode noise is 
a noise which is complementary in respect to the ground in the two 
lines.

What I see in the schematic are three LP filters, two of them (R1 C1 and 
R2 C2) filter the signal in the two lines independently, with respect to 
the ground, while the third one (R1 R2 C3) filters the whole 
differential signal. But aren't we just adding poles to filter the input 
signals? I mean, if I set the cutoff frequency of the two CM filters to 
a certain value, these two filters will just filter the signal within 
the range set. In the same way, the differential filter will just filter 
the input signal within the range set.

    >How exactly do these two filters work together?
    >How can they act on different kinds of noise if they are just LP 
filters?