There is a recipe for the 9 gem clusters that takes two 2-gem clusters and four individual gems. Granted, the 8-gem cluster recipe works first, so it doesn't seem to be exploitable, but it does mean that a recipe for two 2-gem clusters plus 5 individual gems does not give a 9-gem cluster.

I think I have a headache after typing this...

I might need to redraft those recipe combinations (due to the code for those number combinations not really covering all possible combinations at the time of implementation) . Good catch, however!
The sad part about this is that this little derp has been around for several years...