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Yep, my Iberital is terrible for retaining grounds when clean but when I let the insides fill up with old stuff, it's almost zero retention in terms of g in to g out. I tend to run a few beans through before first shot in an effort to ensure as little of the coffee which can actually move is old, and am then happy to avoid regularly cleaning it bar the odd bit of burr cleaning granules.
Scrabble-
Yep, I guess it's one of those things that's come down to numerical comparisons due to the ease of comparison. But the numbers used aren't always representative of what's happening. A good finger in the air comparison is if you have to significantly change the grind, how far off is the intermediate shot between one with all coffee ground and the old setting, and all coffee ground at the new setting. Not useful to plug into a grinder-comparison chart but a lot more useful of an actual measure.
mmccarthy
Maybe not a tablespoon, but a few grams (between 2-3 I'd guess) is the norm in the sorts of grinders I worked with in coffee shops. It's a tricky thing to measure directly and I'd be surprised if the Sage only holds a quarter of a gram. That's lower than both the Niche Zero & Baratza Sette which are the go-to for low retention grinders without spending ridiculous money.
There's also the point that not all retention impacts the coffee equally. The stale grounds left in the chute will have a much bigger impact than the stale coffee that ends up getting compressed and caked in all the nook & crannies. Given that the former will be stale coffee that ends up in your new shot, the latter will not.
I found this to be a pretty decent overview: https://clivecoffee.com/blogs/learn/grind-retention