This issue never occurred to me before. As a pilot and instrument flight instructor, this has proven to be fascinating.
Please tell me you're not a commercial airline pilot!
A lot of nautical charts are in non-WGS datum. Most GPS receivers have multiple datums to choose from but occasionally you come across a chart with a funky datum that is not in the receiver, like the Rome 1950 datum is not in the standard US Navy GPS receiver. Plotting a position requires manually adjusting the GPS position.
I don't think a lot of ATP's would have thought about it either. It's the receiver adhering to a standard, which was selected by the FAA. I just never thought about the possibility of there being multiple standards available. This is just good Technicolor for ground school.
ETA: What's really amazing is running a product like ForeFlight on an iPad, especially with an external antenna. Even though it's not certified for primary IFR navigation, I can zoom into a sectional, terminal, or IFR en route chart or approach plate to the point where the font on the charts almost becomes the same size as the position indicator and it's all dead on. ForeFlight claims 5 meter accuracy, and I can't doubt that; when using the airport diagrams to taxi, what I see directly in front of me sure matches where the airplane icon is on the drawing.