a few seed suppliers seem to do simmilar offers...
i would guess the mixed bags consist of whatever is lying around at the time,
they may have had deliveries of un-even ammounts of seed,
but i would asume the bulk of the mixed bags are made up of strains that arnt as popular as whatever seeds are trendy at the time, so it helps them get rid of slow moving stock.....which dosent nesseseraly mean they are sub-standard, just not popular sellers.

its just good business sence, and if the price is right, then i would say go ahead and give them a try, but i would expect the price of 40 seeds to be less than the average price of 4 x 10 bags of thier medium/low priced "named" strains to make it worthwhile for the customer.

do the maths and see if it seems worth it.
it would be near imposible to identify wether any plants are specific strains, hence why the coustomer should pay less than the average equivelant for taking a chance.

so, for arguements sake, if thier average/lower price for 10 "named" seeds is £30,

30 x 4 (40 seeds)= £120

a fair price for "unknowns" would be about £60-£80