Does the expression, 'must have pp.', have two meanings?
A couple won a lottery but they used the money to help people and didn't spent a dime on themselves.
Here is their conversation.

"We didn't know what we were going to do (at first). After we got it, we wrote down all the family names and then all the charities. We had two pages and we kept adding and adding. We must have given away 90 per cent."

I think 'We must have given away 90 per cent' means 'We had to give away 90 per cent'. Am I right? I know that the must have pp. expression is to show we are certain about something that happened in the past. e.g. It must have been love. I was taught like this.
Does the expression, 'must have pp.', have two meanings?

Question

Does the expression, 'must have pp.', have two meanings?
A couple won a lottery but they used the money to help people and didn't spent a dime on themselves.
Here is their conversation.

"We didn't know what we were going to do (at first). After we got it, we wrote down all the family names and then all the charities. We had two pages and we kept adding and adding. We must have given away 90 per cent."

I think 'We must have given away 90 per cent' means 'We had to give away 90 per cent'. Am I right? I know that the must have pp. expression is to show we are certain about something that happened in the past. e.g. It must have been love. I was taught like this.
Does the expression, 'must have pp.', have two meanings?

Answer

No, sorry. You aren't right, and this construction doesn't have two meanings as you suggest.

'We must have given away 90 per cent' does not mean 'We had to...'..  If they had meant 'We had to', they would have said 'We had to..'

They mean that they don't know the exact percentage for certain, but they are estimating that 90 % is a very likely figure. They are almost (but not entirely) sure that that is the amount that they gave away. The construction 'must have + pp' is used for deductions about the past.

Answer

"Must have + pp" has only 1 meaning that I can think of.  It was explained to me as the construction you use for logical deduction.  You have evidence and you consider it in order to make an assertion about a past event.

In this case, the people were not counting their money while they gave it away and so perhaps did not say to themselves that they had given away 90% of it at the time.  But in retrospect, they have concluded this from the available evidence.

The same rule applies to "it must have been love" (a song title!).  Looking back, the relationship was a loving one though this is a logical deduction from present evidence.  Perhaps the relationship is over now, and they feel differently.http://www.azlyrics.com/lyrics/roxette/itmusthavebeenlove.html