After some quick reading on google it appears this is a common situation and its just accepted. Well my tanks a planted discus tank and I think the difference between 86 degrees and 90 is pretty big and is not acceptable. I mean plants are going to melt at higher temps, etc so I really need accuracy. Did I mention that I have 3 eheim heaters in the tank all set the same and sitting side by side and they go off and on with reliably inaccurate readings as well? I know they're able to be fine tuned but I've yet to be able to fine tune them all to actually go off and on at the same temperature so apparently inconsistency in temperature ain't too big of a deal or else this wouldn't be an issue.
I'm looking at food grade ones I guess based on other people with similar complaints. So far this seems good