Because I added a bunch of bodyfat in the process, and I KNOW my diet is spot on and super clean.
It's easier to add LBM when you're already lean. 15% might be relatively lean, but IMO not enough to bulk. Granted, not everybody has the same genetics. People like your friend might be able to bulk around 15% and do great. But people like me and you (based on your pics) are genetically predisposed to adding bodyfat quicker than the average guy, so we need to pay special attention to bodyfat. I added a ton of strength and some size, but i'm too damn fat now to appreciate or enjoy it! Back to cutting again...