then anyone can predict anything

Doesn't hold much weight :)

btw from google:

guar·an·tee

/ˌgarənˈtē/

Noun A formal promise or assurance (typically in writing) that certain conditions will be fulfilled, esp. that a product will be repaired or...

Verb Provide a formal assurance or promise, esp. that certain conditions shall be fulfilled relating to a product, service, or transaction.