For one thing, it depends on other things that are happening in the background in whatever image processer is using floating point processing. A lot happens in the background in babl/GEGL/GIMP.
For another thing, and this is stretching my knowledge of floating point arithmetic, floating point uses part of its precision to encode the decimal information, and part to encode the stuff before the decimal. Over the range of possible floating point values, at the upper end, there’s less “stuff” available to encode the stuff after the decimal point, or whatever serves for a decimal point in floating point. My apologies, I’ve read through various papers, but a lot of what I read on this topic I promptly forgot as being of no practical use for the kind of editing I do, including forgetting the actual technical terms. But searching the internet for papers on floating point arithmetic will turn up the relevant information.
For a third thing, it depends on the file format used to save the image to disk. See this article:
Hopefully someone with more precise knowledge of floating point will chime in.
Regarding “background processing”, I’ve asked the GIMP devs what the “throughput” precision is, but I don’t think anyone has come up with any formal measurements. I do a lot of testing to make sure that resulting values for this and that seem reasonable, and so far I haven’t had any reason to say “this operation doesn’t produce precise enough results”.