They're static if you want them to be static, they are re-calculated if you want to re-calculate them.
And this all comes down to when PCs level up, nothing more, nothing less. How easy or hard do you want to make it for the PCs to level up? Do you want it the next session, the session after that? The session after that? None of it truly matters. Whether you give them the full calculated total of 1000 xp even though it took them two fights, or two groups of 400 xp (800 total) for each of the two fights... it doesn't really matter. In some number of sessions they are going to level up.
The only thing you are doing by telling your PCs the xp total for the encounter is what you are expecting of them as they play. You give them the full 1000 xp, you're telling them "just get the job done and you'll be rewarded, regardless of how you do it". You cut the xp down and only give them 400 xp for each fight you're telling them "more risk equals more experience, so if you go the less dangerous route you won't gain as much". Figure out what playstyle you'd prefer your players to have, and give them the xp to embolden it. But other than that... how much xp you give them doesn't really matter other than what session they're going to eventually level up.
