I'm confused - that isn't what I understand O(1) to mean. To me, O(1) means only that there exists some constant that bounds the runtime, while individual invocations can absolutely be sometimes faster.
What it really means is that the execution time does not depend on the size of input (n).
What it does not say is how much time it takes to execute, whether it is exactly same amount of time every time or whether there exists some kind of upper bound on execution time.
For example, an algorithm that takes 1/(randFloat()) time to insert an element to the data structure where randFloat() returns any possible floating point number with equal distribution, is still O(1) algorithm, even though there is no upper limit on how long it can take to execute.
According to the definition, if 1/(randFloat()) = O(1) then there must be a constant M that satisfies 1/(randFloat()) <= M * 1. But according to your own words there's no upper limit to 1/(randFloat()), therefore there's no such constant M, therefore it's not O(1).
(In practice on most systems there would be an upper limit since a float can't be infinitely close to zero, but let's act as if that wasn't the case.)
What is, however, clear is that its run-time does not depend on the size of the input. And technically, that means we can find a constant (infinity) that always... But, that is pretty unsatisfying.