What's the difference between actually and certainly?

[Samuel]

Business Idiom in English, Intermediate level

ESL/EFL Test #136 "How to make exclamations", question 9

You could have knocked me down with a feather! You mean to say he's ......... cleaned my car.

(a) certainly
(b) surely
(c) naturally
(d) actually

Business Idiom in English, Intermediate level

ESL/EFL Test #136 "How to make exclamations", answer 9

You could have knocked me down with a feather! You mean to say he's actually cleaned my car.

Correct answer: (d) actually

Your answer was: incorrect
You could have knocked me down with a feather! You mean to say he's certainly cleaned my car.
_________________________

Why can't I use a word"certainly"?

What's the difference between actually and certainly?

Samuel

[Conchita]

actually (SURPRISE)
adverb
used in sentences in which there is information that is in some way surprising or the opposite of what most people would expect:

I didn't like him at first, but in the end I actually got quite fond of him.
I'm one of the few people who doesn't actually like champagne.
HUMOROUS Don't tell me he actually paid for you! You are honoured!

(from Cambridge Advanced Learner's Dictionary)

Certainly means 'of course', which suggests that we are not surprised at the situation in question.