Stores are full of it around here...
Seriously, I think this is just another product effected by JIT inventory. Grape Nuts is "enjoyed" by .0001 % of the population, so it is manufactured sparingly at best by Post. JIT is a great system, but it's Achilles heal is spikes in demand. Grape Nuts demand spikes to .0002% of the population or the .0001% panic buys and there is a "shortage".