Selectivity in acetonide formation from a polyol In my total synthesis lecture notes the above example is given in which a polyol is reacted under acetonide forming conditions. 

The notes point out that this product is the only product, no other actinides are observed. That said there possibly are two other products, both containing two acetonides, so why should just this one di-acetonide form?

The conditions are not very clear, but I assume all steps are reversible so what is actually being observed is thermodynamic product.
Acetonide formation (isoproylidine ketal formation, to be more correct) is indeed a thermodynamic process, and under acidic conditions acetonides will tend to migrate to form the lowest energy product. 

If you look at the penta-ol you've been given, there are three possible permutations containing two acetonides (theres also the possibility of just forming a single acetonide, but these kinds of reactions are usually ran with an excess of the acetone/dimethoxypropane, so its likely to go on as many times as possible):  



The green one (which is the one labelled A in your image) is the thermodynamic product, which becomes obvious if you consider the three-dimensional conformation of 1,3-syn and 1,3-anti acetonides (note that this analysis only holds up then there is no substituent at the 2-position. If there is a methyl group there, as is common in polyketide type motifs, then the acetonides don't necessarily adopt chair conformations):

 

In the syn acetonides, there are three equatorial and one axial groups (minimal strain), whilst in the anti acetonides (which both of the un-observed products contain one of), there are two equatorial and two axial groups, giving a 1,3-diaxial clash across the ring (hence raising the energy of these products).