I don't know if "immeasurability" is the term I'm after. Whenever I search for "immeasurable" I get references to infinity, which is not what I'm looking for.
If I say I'm 74% certain of something or 76% certain of something, although there is a mathematical difference, in practical terms the two statements are identical, i.e. I wouldn't make any decisions differently based on the two statements. However, choosing between 50% certainty and 90% certainty, I would potentially make different decisions.
$$76\% - 74\% = immeasurable$$
$$90\% - 50\% = measurable$$
$$1\% - 0.01\% = measurable$$
$$76\% \div 74\% = immeasurable$$
$$90\% \div 50\% = measurable$$
$$1\% \div 0.01\% = measurable$$
although I might consider there to be a difference between 80% and 90%, but not 16% and 18%.
As an example, if the weather bureau says there's a 74% chance of rain, or a 76% chance of rain, it won't affect my decision about whether to take an umbrella or not.
The percentage only makes sense for events that haven't passed yet. Tomorrow might have a 75% chance of rain, but yesterday it either rained or it didn't, so it can't be expressed as any percentage except 0% or 100%.
Is there a mathematical way of describing this?