The seller will set a price for X amount of pumps that fit in a box. In this case it will be the WP10's.
In this case it looks like Mario has posted prices for two different possible orders. If for some reason he does not meet the required amount per package pricing he can always combine the orders and mix and match pumps. (I've done this twice with the same seller).
I have noticed however a price increase is usually the matter when I have mixed bigger pumps with the smaller pumps. My assumption is because the bigger pumps usually take up more real estate in the box, thus decreasing the amount of pumps to distribute the shipping cost.
But most importantly it is up to the person setting up the group buy because he is in charge of getting the best bang for your buck. And at the same time has to deal with many request that all come in at different times.
Mario has done a great job with the GB's.