Problems about continual pilots in DVB!

Hi,everybody!

In ETSI EN 300 744,it is abviously that the freqency domain continual 
pilot carriers are NOT equidistant,in 2k mode,they are [0,48,54,87,
141,156,192,...],and I can not see any rule! why so?
I think:
The pilots are "boosted",so,
with frequency domain correlation on the OFDM symbol, 
is it possible to extract freq.offset if it is high?
Is that the reason why continual pilots are distributed in a disordered
way?
Could you tell me why?Thanks in advance!