Sorry if this is obvious, I am not a professional. I like to trade 30 year treasury zero's.

I have noticed that the price for a 30 year principal payment is never the same as a 30 year interest payment. The difference is small (~.3%), and I haven't tracked it long enough to confirm if one is always greater than the other.

Can anyone tell me what is going on here? Is there a tax arbitrage at play here? Is it that the principal is considered slightly more guaranteed than the interest payment? (Or vice versa?)