@mummyofthreemunchkins YANBU to be a bit miffed, but YABU to say anything, as it makes you look a bit bitter and petty. Just roll with it and if the lass comes, she comes... You are never going to get the exact number coming that you plan for, so make allowances for that... Like get some food that is non-perishable, so it can be put away for another time/another occasion, if 3 or 4 people don't turn up.
We had parties for our kids when they were school age, and we regularly invited 12 to 15 kids. We would get a yes from most of them (say, 13 out of 14,) and only 10 would turn up. Still an OK amount, but 3 or 4 out of the 14 who said yes, would not come. No reason. No apology.
One time I did laugh though. My older DC (they are 2 years apart,) had a party for her 7th birthday, and we had a McDonalds party. We invited 20 classmates. All aged 6 to 7. They all said yes except 2 who had prior arrangements. So that left 18.
Then one girls mother rang me, and said 'Sorry, Olivia won't be coming to Elizabeth's party, because she feels it's too childish for her. She said a McDonalds party is FAR too immature for her.'
Olivia was SIX. And her mother claimed a McDonalds party was 'too childish.' LOL! (And that Olivia had said so!!!) 
So that left 17 kids, and my 2 DC. They had a ball, and were chatting about it for days (at school...) Elizabeth said Olivia was just glaring at them all. Wellllll..... she was invited!!! 
Also OP, I am wondering (like several others,) who did you speak to? The child or the mother? Or did one of them speak to your daughter?