GIMP Version: 2.10.12
Operating System: Windows
OS Version: windows 8.1 64bit
GIMP Experience: Basic Level

List any relevant plug-ins or scripts:
fuzzy select

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I have an image I want to crop down and fuzzy select + invert selection is leaving behind and some pixels in the selection and also deleting some pixels outside the selection. All of the leftover pixels are somewhat transparent after deletion even though they don't appear to be any different than their neighbors that are correctly deleted. is this just a quirk of fuzzy select? I didn't change any option throughout my selection process besides diagonal neighbors and the threshold level.

Those pixels that are being left behind are probably different colours in the selection.
Try to put your threshhold up a little (Shown in tool options of the fuzzy selector). If you have to, select with the fuzzy selector a couple of times until the whole area is selected but make sure your double square (add selection) is checked (Also in the tool options)

hm, I didn't think that would be it but it is. Strange that gimp shows a pixel is selected/unselected even if it isn't/is. The image I am working on is pretty complex and reselecting parts I can't really see isn't really an option. everything I want is in my outlined selection, is there a way to reselect that whole area for all included colors?

A pixel can be partially selected. The "marching ants" are on the dividing line between pixels that are more than half-selected and those that are less than half-selected. Which means that things can change outside of the ants line.

Post your picture and someone may be able to give/show you a better how to.

You can select more than one colour with the fuzzy select if you click on this in tool options.
For instance a blue sky can be many colours. If you want to selct all the sky you may have to make a few selections. If your area you want to select is all the same colour/hue a higher threshhold should grab all the pixels but beware you don't make it too high or you may encroach into the area you didn't want to select.