Imperfect subjunctive question

[jsjavensb]

Hello. I've come across an instance of the imperfect subjunctive in a book that I'm reading but I can't figure out why the writer used that tense because the example doesn't fit with any of the rules that I know for imperfect subjunctive. The book is written in Ecuadorian Spanish.

Here's what I read (the confusing verb tense is underlined):

[the situation happening before the quote in question is that a man has come home all excited and happy because his baby has just been born. He goes into the room where his wife and sleeping baby are. He picks up his baby and becomes sad thinking about the struggle that the baby will have in life and the text says:] <<"llegaste a este valle de lágrimas"--dijo con enorme tristeza que constrastaba con la bulliciosa alegría que demostrara minutos antes>>

So when I read it, I wonder why the writer didn't just use preterite or imperfect indicative (demostró, demostraba) and I can't see how the use fits with any rules I know (the rules I know are what is listed in this webpage: https://www.spanishdict.com/guide/spanish-imperfect-subjunctive)

If anyone can help me anlayze this, I would really appreciate it.

 

Thank you.