So I have a few GE undercabinet fixtures, the sticker in the lamp compartment it says to only use F17T8 or F18T8 replacement lamps, yet the GE magnetic preheat start ballast inside states to only use F18T8 lamps. Why does it say this if the lamp specs of F18T8 and F17T8 lamps are very different? I am asking because F17T8 lamps are way easier to find and are cheaper too.

The ballast will only drive correctly the F18T8 (or F20T12, if it would fit in the luminaire). the F17T8 have way different voltage and current specs

https://www.lighting-gallery.net/index.php?topic=12721.0

Yes, I know F17 and F18 T8 lamps have totally different specs; the sticker in the image says to only use F17T8 or F18T8 replacement lamps, while the ballast specifies F18T8 lamps. See the attached images.

Think how you would understand it if there were 2ft and 4ft versions of the luminaire, and the label would read use F18 or F36 lamp

For the user, this would still possibly confuse (a random user who does not know lamp naming conventions, so would go to the shop and ask for F18 OR F36, and only then find out that they are of different length), but still he won't be able to actually fit the wrong lamp

For whoever edited this label at GE, this might be a short hand (and not very well thought of) way to use one label for all the lineup of luminaires

I think something similar might have happened here