Crash One
Please show me where CAP413 states 'Roger' = understood.
Your instructor was showing good airmanship, in my opinion, in challenging you.
There are many reasons one may get a 'roger' from someone who received the whole transmission, but did not understand, including the humn factor you mention
Before this gets out of hand and into a DFC type argument, my point is simply that one should know the meanings of standard phraseology, if you are going to use it.
And that meaning should be unambiguous. Roger means you received the whole message, nothing more.