I honestly don't know enough to guess at alternative explanations myself -- I can only go by what they told me, which was "incidental calcification" with the explanation I posted above.
On the full-size version (and the alternate view for his spine X-rays), it's easier to see that they are not identically shaped or sized, but they are still really weird looking.
Whatever they are, they don't seem to be hurting him and they're not indicative of any disease (as far as I know), but... it is a strange finding, I agree.
ARCHMX TDCH Pongu the Insane, CD-C, RE, RL1X6, RL2X5, RL3X2 (GSD mix, b. Apr 2010)
Crookytail the Tigerwuff, RL1, ITD (Akita mix, b. Jan 2011)