Why are you such a @#*%#*#!? Autism

Why am I finding it hard to convince people that what I do is down to autism? Let me set up a scenario...

Me and my wife are splitting up, but one of the boys had a birthday coming up and i was skint. I wanted to get him a present and a card. I asked her for some money and she obliged. I picked up some fun items, something silly, a book he wanted to complete the set, a card and a gift bag to put them all in. I write the card out "from Dad", because, after all. we aren't together anymore.

I gave him the bag and he opened the card. He showed it to his mum, who went ballistic because i didn't write it out to both of us. She hadnt managed to get a card for him herself, of which i wasn't aware (she even give him cash to get what he wanted that i wasn't aware of either).

Much later, my partner asked why I wrote the card out just from me, but says, "and don't you dare say autism!"

Who is to blame here? Is anyone to blame? Is having autism a good excuse, assuming i misunderstood the circumstances and was not aware of what had already been arranged?