SnakeLab, you probably already thought of this, but was the YB mama a virgin?

The only other thing I can think of would be that she had been previously bred by something other than the ivory, and retained some sperm from that.

Aside from that, it will be interesting to see what that little girl produces -- as krmn22 above me suggested, maybe she will throw something funky into the ivory mix, like some of the other "normal" looking variants in this complex seem to do (specter, etc. -- and yes, I'm sure there are ways to differentiate a specter from a normal, but danged if I can tell what they are!)