See, you must use the concept of principal functional group only for nomenclature purposes. Here the compounds you mentioned in your question have two different groups in addition to carboxylic group, i.e., ketone and aldehyde. These compounds are chemically different. They show the traits of ketone as well as aldehyde in the respective compounds. So it is better to call them functional isomers. The answer in your book is correct. No need to worry about that.
If you have any further doubt, feel free to ask me any time. I will be glad to help you.