While inherently closely related, there are significant differences between "taking an exam", and "achieving a certification". Exams are a means to an end, that end being a certification, but they are distinct things.
You must take exams to achieve certification; however, not every exam results in a certification. For instance a candidate who takes exam 101 will not see a certification on their records because exam 101 does not result in a certification; you must pass both exam 101 as well as exam 201 (or 202) to achieve our first certifications (F5-CA, BIG-IP or F5-CSP, Sales respectively).
Another difference is that exam validity is for 2-years, but can ONLY be renewed by retaking and passing the same exam. Exam validity has no bearing on certification validity.
Certifications are achieved by satisfying the requirements which may include additional things beyond passing an exam; although passing an exam or combination of exams is the most common way to achieve certification. Passing an exam does not confer a certification, but may earn a certification.
While certifications also have a 2-year validity, unlike exams, certifications may be renewed in other ways beyond passing an exam. For instance, if you achieve an F5-CTS certification, your prerequisite F5-CA will automatically be renewed even though your 101 or 201 exam results may have expired many years before.
In summary, exams are only a means to achieve certification, they are not certifications in themselves. In addition, the validity of exam results and certifications can differ greatly depending on where you are in your certification journey; however, certification validity is independent of exam validity and is paramount.