A little help with the usage of 「……わけだよ」

Question

I came across the following sentence in my dictionary :

下手なわけだよ，習い始めたばかりなんだから

Because of the absence of context, I imagined one by myself. Suppose I ‘m cooking unskillfully and a friend who sees that says ,“お前料理下手だな”. Then to make excuses I say, “下手なわけだよ，習い始めたばかりなんだから.”

The above is the context I imagine, but a native speaker said that in this case in which I want to make excuses for myself, わけ is not suitable, and はず is OK.He didn’t explain more.

I know the nuance between the following two sentences:

A.彼は七年間も日本にいたから、日本語が上手なわけだ

B.彼は七年間も日本にいたから、日本語が上手なはずだ

I just want to know why the usage of わけ such as in sentence A couldn’t be used to make excuses for oneself. It’s very anti intuitive to me..

Thanks in advance, and sorry for my poor English..

Answer 1

In my opinion, it should be 下手で当然だろ？ (It's natural that I'm poor at it, isn't it?) , which can barely be interchangeable to 下手なはずだろ？, but not はずだよ.

Neither わけだ or はずだ are really correct because わけだ (No wonder that's that) can only work when you provide information the listener isn't aware of or when you hear the reason you haven't been aware of, and はずだ is only synonymous to the latter beside assumption.