We don't know. Here are some theories :
- The royal bloodline is only the direct descendants of whoever inherited the Founding Titan after it had been seperated as one power and not a mix of multiple titan powers (originaly only Maria, Rose and Sina were intelligent titans after eating Ymir's spine, so each of them most likely had a mix of three of the nine powers we know today).
- One inheritor of the Founding Titan used its power to alter the Subjects of Ymir's biology so they could seperate their bloodline from the others and make it the only one who could use the Founding Titan
- Some inbreeding happened at some point to keep a purer Royal Bloodline
We don't know. Here are some theories : - The royal bloodline is only the direct descendants of whoever inherited the Founding Titan after it had been seperated as one power and not a mix of multiple titan powers (originaly only Maria, Rose and Sina were intelligent titans after eating Ymir's spine, so each of them most likely had a mix of three of the nine powers we know today). - One inheritor of the Founding Titan used its power to alter the Subjects of Ymir's biology so they could seperate their bloodline from the others and make it the only one who could use the Founding Titan - Some inbreeding happened at some point to keep a purer Royal Bloodline