Are you saying 2.1% -> 2.1% (no change) would be a 100% improvement, and 2.1% -> 0.21% is a 10% improvement?

290%

or

190% improvement

but not

290% improvement

No, this would usually be called a 190% improvement. (6.1 - 2.1) / 2.1 = 1.9

Similarly, 2.10 to 2.31 is usually called a 10% improvement, not a 110% improvement.

You guys are right. I confused myself with the wording and thought too much about it.

Thank you for correcting me.