People knowledgeable about cats, why do they vomit?

This question is in the General Section.

I am seeking input only from people with knowledge of cats. All others will be flagged.

My cat is completely healthy to all appearances. She has a healthy appetite. She drinks water. She only uses her litter box and never goes outside it. She acts completely normal.

She vomits about once a month. It appears like a lot of liquid and a smallish amount of gooey matter the same color as her food.

She’s an indoor cat.

Thanks.