The thing to remember is that bearded iris are highly hybridised from a fairly diverse set of species with often very different habits, most of the original species can interbreed but not all of them do so readily. If you look into the bigger/more well documented hybridisers you'll tend to find they often use a pool of related iris (often that they have bred over years) and that they tend to produce more over time and relatively few early on....what's happening there is the fertility related genes are being skewed towards being like each other/compatible within the pool they are hybridising from. If on the other hand you're crossing two iris from completely different sources it's more a case of rolling the dice about whether the genes connected to fertility will be readily compatible or just difficult. It's the same thing that happens in the majority of highly hybridised organisms (roses is probably a good and fairly well documented example), with a lot of species in the genetic history of them, the genes of fertility aren't always going to be highly compatible because the wider genepool isn't as highly skewed.
