I have been getting only the Spellspine Dragon every time for the last 11 attempts in both the Breeding Den and Roost when I breed pure Green with pure Purple. Is it just my luck or is there something else at work? Anyone else experienced something similar to this?

A similar thing happened to me last week while I was breeding pure Green with pure Yellow, I got eight Nymph Dragons one after another. I didn't mind those because they gave me 320 essences when I disenchanted them, but the Spellspines are a waste of time.