This is what I thought... there are brits in the comments saying they have used this phrase in this way many times.

That said, it is a little odd that it made it through 3 reviewers, none of whom objected or got clarification — or if they did, it's not on the record. (edit: which/whom)

Using the plural for "data" ("Where are they?" it says) increases the likelihood that "make up" just means "do" in that context.

That it made it through the reviewers suggests the rigor of the review process. :-) Was this a major journal?

I also wondered if there was something less nefarious in the explanation. Was it "Falsify some results" or "Toss up some BS about this topic". The latter is bad, but part of the academic paper business. The first is career ending.

The student is now a Postdoc in Brazil and the PI has since moved to Western Australia for another position. These moves happened in 2011 and 2012 respectively, according to their linkedin profiles. So unless they're still working on the project, it's unlikely the phrase 'just make up' means to put together.. rather it's more likely that it means to fabricate.

Basic elemental analysis is very easy to fake, and very hard to demonstrate it was faked.

Since the prof studied at the university of Neuchâtel and Salerno I guess it's save to assume he's not a native english speaker. So I think it's possible that it was just sloppy wording.

I'm curious what his statement is going to say.

"just make up" makes this interpretation unlikely.

Additionally: it matters just as much how the grad student interpreted the statement, as how it was intended.

Even if meant with good intention, which I doubt, the use of language is in itself poor because the intent is not clear.

"hey, lazy grad student, we're in a hurry. Just make up the table from the data you have, already."

I happen to personally believe that the guy is guilty of attempting to fake the data - but it is certainly a plausible explanation that should not be discounted.

Only if the task is difficult. If it's a simple task, the phrasing is fine. I have no idea how hard this particular thing is, though.