I've worked in a few places where the space was removed to guard against bad line breaks, where the number could end up orphaned at the end of a line and the unit abbreviation was at the start of the next one. Non-breaking spaces were often too fiddly to deal with, so removal was a decent compromise.

I'm a big fan of the Economist style guide (and Orwell's essay Politics and the English Language, which informs the guide) and have used it shamelessly as the basis for guides I've written in newsrooms over the years.

I think The Economist would optimize for conciseness of the representation instead of just following SI. Ex: 1.1t of a thing instead of 1100kg is better in the middle of a paragraph.