Note the free version of malware bytes only runs when YOU manually run it. If you never run it or only run it now and again it is not doing you much good.
The paid for version is running all the time (in the background) so is monitoring your computer all the time.This means it spots malware as it arrives on your computer.
The free version does NOT spot malware arriving as it is not running. It will only find the malware when you choose to run malware bytes.