That's how scientists speak.
Evolution is a theory that is well supported by evidence, but not necessarily the full picture.
Gravity and magnetism are theories that are well supported with evidence, until/unless a better theory comes along, like relativity and quantum mechanics.
Scientists believe that planes can fly, beyond a "reasonable" doubt, but sometimes they fall from the sky and the explanation why is very complicated.
We send people to the electric chair based on "the evidence strongly suggests."
Sometimes its the best you can do.
Only "faith" can be so rock solid, because it is not based on anything real and is not subject to testing.