Preapproved but now asking for higher downpayment

I was pre-approved for a 20% downpayment loan on a $725,000 house. Realtor offered below asking of $700,000 and was accepted. I just got the lending docs and they indicate a 25% downpayment. Called the lender and said they will only lend to me with a 25% downpayment. Luckily I have the money, but is this normal? The pre-approved downpayment was $145,000 but now I will be paying $175,000. I'm just wondering if this much of a difference is common (I know preapprovals/prequalifications aren't guaranteed).