Long live the .17 Rem and 25 Horn HP. This was back in mine's OEM stage...I only have the action from this anymore. Complete makeover reality, it got.
These two dogs dropped instantly at about 75 yards and 100 yards IIRC.

[Linked Image from i.imgur.com]
[Linked Image from i.imgur.com]

This is it now:
[Linked Image from i.imgur.com]



Even with just a sample of two, I think you've got something else going on, as two in a row is a bigger statistic than just two in general, but who knows...
3500 FPS Muzzle Velocity at 20 yards should be plenty of horsepower. Maybe Hornady has changed their metallurgy in their 25 grain HPs? I am a serious hoarder, so haven't bought any for quite a while. Maybe something to do with your particular barrel?