What a conversion rate is, and what counts as one
Conversion rate is conversions divided by visitors, as a percentage. The arithmetic is trivial and the definitions are where it goes wrong.
The first question is what a conversion is. A purchase, a signup, a form, an add to cart: a store usually has several, and a rate quoted without saying which one is a number nobody can check.
The second is what a visitor is. Sessions and users give different rates for the same behaviour, because one person can return several times. Neither is wrong; mixing them is.