Hah, you're in the same shoes as I am with the distance measurement... I correct it by 1% in my log now, may be conservatively estimated, last year on my motorway trips I saw the same 1.5% difference as you, and since I have these new tires the measurement may be even more off (my commute used to be 24.6km at the end of the last pair of tires, now it 'is' like 24.4km). And when we ride together with shiNIN, her bike always measures 1-1.5% more.
And I still couldn't measure it by GPS, damn my phone...
Anyway, how the two bikes' FE compare? Heck, if I had more bikes, they'd be a bit more different
(The next one would probably be a very light dirt bike. Or a little CBR, like yours, or even the 125, I love its 'KTM orange' theme
)