What comes to your mind when someone says the word, "egg"?
I bet it's a chicken egg! Even when you google the word, the entire google list is filled with chicken eggs. What about duck eggs, ostrich eggs and penguin eggs?
We often associate and assume a general term to be something specific, something that we always see and is familiar to us, something close to our hearts. Also, as a Chinese, I use the term 鸡蛋(chicken egg) so commonly till I never use the word 蛋 (egg) by itself anymore. So even if I'm referring to a duck egg, I'll go like, "鸡蛋，Eh 不是！鸭蛋！" (Chicken egg, EH NO! Duck egg!)
Lesson learnt: Bear in mind that although this often is a right assumption, there are times when you may be wrong.
In short: Don't assume.