We use COULD to talk about things we are able to do generally. We don't normally use COULD for something that happens on a particular occasion in the past.We use was able to or managed to do"
Source: Oxford Practice Grammar
Now, in the same book, in the PRACTICE section, there was an example below:
Last year Robert COULD beat his brother at chess. But he can't beat him now.
I have a question: Why does the writer use 'COULD' instead of 'was able to' or 'managed to do something', because the example above is not general situation it is a particular occasion.
I feel that although that situation happened last year, it wasn't happening during a whole year. For example, every week, every month or every day. Playing chess and beating her brother just happened on a specific day of the year or a specific month of the year, not the entire year. That is why I think it is not a general situation.