Batteries age, even if not used. FWIW, NiMh seem to fare better when used. A half dozen full cycles (0.9-1.0v > full) will likely help, but won’t make them young again. Like I said, try it.

Resistance readings on chargers like that are “approximate”. Due to the slider mechanism they are not highly accurate, or repeatable. They are better than nothing, for sure.
HKJ test found that unit to be decent:

[The internal resistance values are only about half of actual resistance, but they are fairly consistent.]

Is this your battery?
http://aacycler.com/battery/aa/duracell-ultra-2500/
Note the rapid rise in resistance, and the drop in capacity. The site is interesting to look at the long term change of many different cells.
Compare that to Eneloop: Panasonic eneloop · AA Cycler

The increased resistance/capacity drop happens to them all. It just depends on how soon and what kind of use pattern.