Renaud, seigneur de la Ferté - Birth year makes him about same age as his son?

I'm following a line upward. Renaud's birthdate is ca. 896. His son Achard's is ca. 892. Based on Achard's son Nocher I's birthdate of 922, I don't think Achard's is that far off. I realize circa leaves wiggle room, but wow. That's a pretty hefty wiggle. Does anyone know what the ca. 896 is based on?

Thanks in advance.