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.