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

Doesn't the higher arc voltage of the F17 cause the ballast current to drop?
So the ballast could deliver the 0.27A for the F17 lamp and something around 0.33A for the F18T8. Assuming about 15V drop on the ballast wire resistance, we are talking abou roughly 97V vs 81V drop (F18 vs F17) across the inductive part, plus a bit larger degree of saturation for the core.
And the 0.33A is not that far from the 0.365 rated current for the F18.
And because of the thing is not governed by EU standards (banning all ballast not driving the lamp at its full rated light output), it could be the way the ballast is designed to serve both (and even some more) lamp types.

Won't F17 arc voltage be a bit too high to be stable with a choke on 120V +-10% ?

I tried both lamps in one of my fixtures. 
F18T8 0.32A 52% PF
F17T8 0.28A 61% PF

and another which underdrives lamps.
F18T8 0.24A 51% PF
F17T8 0.20A 63% PF

The only issue I've encountered is difficulty starting in the second fixture.  The current is too low to preheat the cathodes adequately, though it worked better when the lamp was new.  I'm actually still using that fixture with the F17, but I've fitted it with a Pulsetarter.  I've never had stability issues once lit.  The F17 never shimmers or drops out.

@NeXe Lights, my hunch is an F17 will be fine, but it might wear out quicker especially if frequently switched.