BPP 4.14 withdrawal and revocation of offer

Hi Mike, can you please help me understand the difference what I have missed in the below question about withdrawal and revocation of offer? This is BPP Practice book 4.14.

4.14
Esme makes an offer under the UN Convention on Contracts for the International Sale of Goods to Mildred with the stipulation that acceptance should occur on 31 March at 12. noon. On 25 March, while Mildred is considering the offer, Esme tells her that it is no longer open to her.
Which of the following is an accurate statement of the situation at this point?
• Esme has made an effective withdrawal of the offer
• Esme has made an effective revocation of the offer
• Mildred may still accept the offer on 31 March/as it was irrevocable
• Midred may still accept the offer on 31 March but Esme may reject her acceptance

Correct answer is: Mildred may still accept the offer on 31 March as it was irrevocable

My interpretation was: Esme made an offer and Mildred received it, therefore withdrawal of the offer is not possible. After the offer was already heard, Esme changed her mind so she has revoked the offer. Where did I miss that this is an irrevocable offer that cannot be revoked nor withdrawed?

Thanks a lot!