its funny cos you only ever hear the 2 things together, never says just 'wean at 6 months' on its own, only when mentioned alongside excl bf'ing...
When DS1 was little I was told by several people that he wuld need weaning earlier as he was bottle fed and he was a big baby on hungry baby formula, so i weaned at 4 months.
DS2 is nearly 3 months and bottle fed too but am going to wait till 6 months this time if thats the guidelines. He is not as big as DS1 and drinks much less milk so should be able to hold out for longer!