We are already domain experts on the research edge for whatever topic, and writing to our peers. What we know is already not obvious to non-experts. For something to be surprising to the domain experts too, that's where the novelty points start, and why we are taking time to write/read. So, clever!
There can be a difference between results and consensus expectation, but there can also be a difference between results and personal expectation (e.g., results do not support hypothesis). In the absence of explicitly calling out who holds the expectation, there is either context that makes it clear (e.g., the audience is experts in the field and the expectations are widely held) or the use is referring to the author’s own surprise.
Simply claiming they are not equivalent is not correct. They are equivalent in the absence of context. It is only when context is considered (and just being a scholarly article isn’t necessarily enough context) that they might be considered non-equivalent.
"We believed this surprising thing might be true. To our surprise, nothing unusual happened."
What's generally surprising is not the same as what is surprising given the researchers' prior expectations. (What is surprising also changes over time, as a result of those experiments!)
“To our surprise” means only that you were surprised by it. It’s more humble because it allows for a reader to be smarter/wiser and not be surprised. “I thought Frodo was going to die at the end of the Return of the King, but to my surprise he did not.” “Surprisingly” wouldn’t be correct here since not everyone would have made that prediction.