Actually, I was surprised to see Sri Lanka A defeat Australia A in the second ODI because as far as I know that Sri Lanka A is a much weaker team than Australia A. In this case, I thought they would be whitewashed in this series.
Except for the second match, I have seen them do nothing but perform poorly in all the other matches. And so I had no expectations that Sri Lanka A could do anything good against Australia A. And at the same time, I will also say here that I do not see any hope for the Sri Lanka A team in the upcoming Test series either.
It is totally understandable why you were surprised when Sri Lanka A beat Australia A in that one day match. Australia A is usually stronger team and Sri Lanka A played poorly in all other games so that win really looked like fluke. Everyone likely expected Australia A to win easily and that one victory probably did not change your overall low expectations for Sri Lanka A especially looking ahead to Test series. Unfortunately they lost in third ODI but they played really well in 2nd ODI. And now they are playing Test match and they made 272 runs with all team out. They are looking little bit weak but we can not say anything at this time.