What is the difference between "in conclusion" and "by way of conclusion"?

I was just wondering this very question myself! I think that "By way of conclusion, I will summarize my main arguments" means "As my conclusion I will summarize my main arguments."

But "In conclusion, I will summarize my main arguments" means, "Here is how I will conclude: by summarizing my main arguments."

The difference is subtle, but I think the first but not the second suggests that there are other good ways (perhaps better ones) of concluding than by giving a summary, and instead of concluding by doing one of those, I will conclude by giving a summary of my main arguments. But the second one just says, This is what I'm going to do:

My real question, though, was how this meaning could be derived from the more general meaning of "By way of...."

To my (American) ear, "by way of conclusion" means: "This is what I am offering as a conclusion". It may imply that this is not a real conclusion (in the technical sense of "given the arguments I have cited, I conclude that…). It may not be a "conclusion" at all, e.g. "By way of conclusion, let me mention a funny story I came across recently…".

When I as a reader encounter the phrase "in conclusion", I have every right to expect that the author will present a conclusion to, or at least a summing up of, the arguments and ideas they have presented in the article.