Probably because 1250 does not divide by 4

(at least to a whole number)
1260 / 4 = 315
Edit: Oh, with
my army! Well, for wood elves I'd take a bunch of dryads led by a branchwraith (since a treeman ancient is too expensive at 325 to fit in 25%). And for my troll army I'd take... well, trolls and the like

. Probably something like:
Branchwraith, Annoyance of Nettlings, Cluster of Radiants- 115
14 Dryads- 168
14 Dryads- 168
8 Treekin- 520
Treeman- 285
Total- 1256
Throgg- 175
8 Trolls- 360
8 Ogres, Chaos Armor, AHW, Mus, Std- 390 (If I didn't need the ogre banner I'd also consider swapping this for 6 naked dragon ogres, same cost)
5 Hounds- 30
5 Hounds- 30
Dragon Ogre Shaggoth, AHW- 275
Total- 1260
I would definitely bet on seeing a level 4 caster accompanied by 2 units of glade guard, and probably some dryads in support for the wood elves. I just faced this recently (only at 1000 points) and I just went for the loose units (a unit of dryads on its own, glade guard scouts, wildriders) and avoided the long walk towards all those spells and arrows. If you're not mobile enough for that, you should still be able to shoot down targets of opportunity and pounce on a few units, while preserving your own from far from his shooting line.
For the
WoC players you will probably see a lot of nasty blocks, like marauders and warriors, and perhaps a level 4 caster as well. So I would bring strong magic defense (should be easy for dwarves

) and some stuff to soften up those blocks that are coming to you, as well as a good counter-charge unit to wipe them out (to get the points, since you won't get anything for just whittling them down) when they do get to you.