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Daffynition:" Daffynition:" FFIW, I recall taking their temp with both the glass/mercury as well as the digital.
...the digital consistently ran 3* over the glass/merc in comparision"

Sorry I didn't get to answer your question about instant-read thermometers, but the hour was late.
Mercury thermometers are a known accurate read.
Electronic "instant-read" thermometers are subject to getting out of calabration and giving errors, especially after long-term usage.
The best way to compare readings would be to pour a glass of water and place both in the glass; the water temperature should be the same.(ie: within peramatures like 98 * )
If there is a difference in readings , I would trust the mercury thermometer; if they differ, it would suggest that one is out of calibration (more likely the eletronic).
I would them mark the storage box/container that the thermometer came in with magic marker noting the discrepency (ie: = or - * )
Depending on price, you might send it back for replacement, or recalibration, or just buy another.