What exactly does "not perhaps" mean? I encountered it in Smith's The Wealth of Nations
In Chapter I. Of The Division Of Labour:
The effects of the division of labour, in the general business of society, will be more easily understood by considering in what manner it operates in some particular manufactures. It is commonly supposed to be carried furthest in some very trifling ones; not perhaps that it really is carried further in them than in others of more importance: but in those trifling manufactures which are destined to supply the small wants of but a small number of people, the whole number of workmen must necessarily be small; and those employed in every different branch of the work can often be collected into the same workhouse, and placed at once under the view of the spectator.
It seems to be along the lines of "probably because"... can anyone find a reference or other example of usage and history of the phrase?