Minecraft runs in "ticks". Effectively, everything is "round-based" and updated x times during a second. Quite all games work like that, though their tickrate/framerate differs.
In case of minecraft, the game is hardcapped at 20 ticks per second. This means, if you FPS >= 20, you will run at these 20 t/s. If your computer is slower, however, your minecraft will run in "slow motion", aka less then 20 t/s.
The reactor uses a lot of CPU and thus only calculates it's inventory every 20th tick (= once per second, on a normal-speed computer), whilst saving the "produced energy" into a variable and emitting said energy every single tick (like a normal generator).
If a Reactor produces 120 EU/t, it does emit 120 EU every single tick. It does, however, only update itself every full second.
If you PC runs slower (<= 20 t/s), your whole MC world will run slower. If you, f.e. got 10 FPS, all machine operations will take 2x the time.
You can watch this interesting effect when playing on an overloaded server. I once played on IR with a dupe-bug lagging the crap out of the server. Smelting one item in the Electric furnace took ~30 seconds realtime, because the server itself was running at <5 FPS. Happened QUITE some time ago, somewhen around IC1.