I currently have a flag weight of 531.7108. Is it just me, or are the 4 digits of precision after the decimal point just a tad unnecessary? Borderline bug I think.
Below 500 flag weight, flag increments are much larger, so we only show an integer.
Once you reach 500 flag weight or higher, flag increments will raise fractionally toward 750, so we show a double.
It is possible for your flag weight to increase by .001 once you start approaching the maximum of 750.
It was actually intended to behave this way, but the actual implementation is such that the increase is always at least 0.1 for flags on questions and answers.
I noticed the same thing, but I think there's a good reason. As the flag weight asymptotically approaches the limit (750 at the moment), each increment will be smaller and smaller, to the point where you might need 7 significant figures to see it change. Of course, you could get away with having fewer sigfigs for lower values, but that might be making it excessively complicated. Just having a consistent 4 decimals would handle nearly every case, even if it's sometimes ugly.
No it's not a bug. You should read this question. And here is a quote that describe reason why we have those 4 digits:
flag weight now goes to 750; deputy badge still 500; above 500, growth is sub-linear