Raising needle to correct hesitation and low speed surging

I recently corrected off idle hesitation and low speed surging on two bikes, an 81 CB650 and an 82 KZ305, by raising the carburetor needles about .080".
Carburetor jetting was stock and stock bikes.
My question is why would this be necessary? Could slide springs become weaker or diaphragms less pliable as carburetors age?

To me this sounds like a lean condition in the pilot system. Maybe raising the needle enriched the air-fuel mixture by allowing fuel to be metered from the main while at low RPM/Part throttle.?

It could be parts are not what they were when new. It could also be the bikes were jetted lean from the factory and now that fuel is not what it was in the 80s could also be playing part in this.

I've had to do this recently as well, on a bike that was running perfectly fine prior.

When I originally jetted the bike 13 years ago, i was interested in jetting it for fuel economy in the part throttle position. So I went about as lean as it could go while still running perfectly smooth. In the last few years it had started to run seemingly leaner. I suspect the difference is that I used to be able to get 100% gasoline. Then it went to 5% ethanol. Now 10% ethanol is about the lowest I can find around me.

For me the change was about .2 to .3 mm on the needle. That is only about .007". Your change of .080" is like 2 mm, which is a huge change.