Hi, sorry for a newbie question!

I have just startet to explore Nxt a little, downloaded and installed version 0.4.8,
bought some Nxt to have something to play with (I have 558 NXT),
and now I have the wallet open and running since about 3 days (localhost:7875).

Currently it shows active peers=131, recent blocks=2588 and my question now is:
Why the number of days telling me when I can generate the next block is jumping up or down all couple of minutes?
It startet first telling me in 5 days, later in 144, then about 500, later about 300, back to 90 ...
currently it shows in 114 days 12 hours ...

From what it depends on?

Cheers,
BitWalker