csc v2 新版 精编教材 滴24章

P123
ACCRUED interest
“when an investor purchases a debt security, the ACB is the principal amount less accrued interest”
但是下面的例题中投资者支付了：1010元（principal）+10元（accrued interest）=1020元，however, ACB is the $1010 only.
请解释一下，看的很糊涂