We spent a lot of time in the early days staring at SVGs to try to understand what worked and what didn’t. The Glowforge software has heuristics and workarounds, some of which are triggered by fingerprinting the file to guess what broken software created it.
Illustrator, in “Save As… SVG” mode, uses “px” as the units, where 1 px = 1/72 in. This is only Illustrator’s opinion, not enshrined in any specification, and is therefore subject to interpretation. The only reason this works is that the Glowforge software sees the comment that says “Illustrator made this” and knows about this assumption, so it interprets the file accordingly. But as we see in this thread, trying to bring it into different software that has different interpretation of what a “px” is can be problematic.
I much prefer the cleaner SVG produced by Illustrator’s “Export”, which appears to be a completely independent implementation from Save As. It produces (as long as you uncheck “Responsive”) real-world units like “in”, which can’t be misinterpreted.
Unfortunately, “Export” SVG has been historically buggier and that’s why there’s a lot of superstition around only using Save As. I have only seen one of those bugs actually crop up on the forum, and it was in an older version, but it’s worth noting that if you use Export, you’re not following the instructions provided by Glowforge and in some sense forfeiting your chance to complain to support if it doesn’t work.
In the course of trying to understand these behaviors, I have read the entire SVG specification, and may have had a few mini-strokes as a consequence. It is an interesting piece of work, particularly the parts that talk about units and coordinate spaces.