If you're decoding them on your titan, crest of the alpha lupi is the only titan exotic available to decrypt, so it's highly likely that you'll get one. Sometimes I'll get a random one for my hunter or warlock, but not often.
If I decode a chest on my hunter or warlock I see much more varied results.
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Its my hunter. I decode on him because everything i decode on him has been 310. But still, 8 chest engrams out of 9 that drop? All 4 primary last week were hawkmoon? All 3 chest were alpha lupi? That's garbage. Not 1 of them was even for a titan.