Because it has 167lbs less guns that the A-8.
I guess for the same reason that the Bf-110G-2 has an ENY of (I think) 20. Whatever formula that HTC uses to come up with the initial ENY value seems to weight ground attack pretty high. Probably nobody has brought it up since few people fly this variant. (Alt weenies like the Dora, furballers like the A-5, cherry pickers like the A-8. Not sure who would pick the F-8.)
I suspect that if you made a polite post with a header like "HTC: Suggested ENY value for Fw190F-8" and gave them the value you thoughtit should be (about the A-8's I would think) and the reason why, they would change it.