i am confused

I interviewed with a company and they gave me an offer a couple of days. However, when the HR verbally told me the offer, I explained some concern about the compensation, such as vacation. HR insisted to send me offer overnight and told me I can let her know my concerns after I review the complete offer. I called a day later and negotiated for better vacation, increased salaried, etc. She said she would get back to me the next day and verbally walk me through the revised offer again and then send it to me overnight. The next day, she didn't call. Instead, she sent a letter to revoke the offer through a messenger service.

I am a bit confused. Is that because they are not sincere to start with or i did do something wrong, so that I can learn my lesson for future.