Angiosperms (broadleaves mostly) have a different idea about supporting branches. They build wood under the limb to prop it up. Gymnosperms (pines, etc for lack of a better term) support branches from above generally and build strength through tension wood.
Each has its advantages but obviously you see few really long limbs on pines... And nothing spreads like a mango tree...
Anyway, the wood layers directly under a limb will usually show a series of oblong wrinkles arranged in a Vee pattern from the sides of the limb tapering down the trunk. Enough of them and you have a pretty neat pattern and they usually build rows of these to the inside. When the tops are sliced off them they produce a series much like a bunch of small oyster patterns.
The grain changes are what create chatoyancy, or cats-eye effect. For some reason Browning seems to really like this particular pattern for Gran Lightnin Citoris. I have two with exactly the same pattern and have seen many more.
I agree you got quite lucky though a fully curly tree would have made you considerably luckier!
Stacking the wood even higher would be better and would give you back some shed space. The extra weight will help control the warping.
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