Using 47mm x 47mm posts for a wire fence / Wickes Anomoly?

right, looking to build a 4/5 foot wire fence for the chickens as they are destroying the main grass area the buggers!
As its only green chainlink fencing Im thinking that 47mm x 47mm posts will do the job, especially as they are a lot cheaper then the 70 x 70 mm post at 13!!
If my proposed size is ok (i.e. 47mm x 47mm) why would wickes list one as a fence post at 10.99 and the sawn treated version is only 3.85?

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