Why do my In-game item ID's differ from whats in the ic2.cfg / ic2_map.cfg ? (every item id increased by 256)

EDIT: It just dawned on me that each item id is being increased by 256. is this normal?
I also tested Buildcraft & some others and discovered they are doing the same thing.

Minecraft 1.4.7
OptiFine_1.4.6_HD_U_D3
minecraftforge-universal-1.4.7-6.6.1.524
NotEnoughItems 1.4.5.1
ReiMinimap_v3.2_06
industrialcraft-2_1.115.221-lf (also occurs in build 170)

Fresh install, making sure no previous ic2.cfg exists, and a brand new world.

Block ID's match the ic2.cfg file perfectly, but the Item ID's are all different.

items in ic2.cfg: 29822-30000
http://dl.dropbox.com/u/1315574/IC2.cfg

Actual assignment in game: 30078-30256 (despite the ID's in ic2.cfg being unused)
http://dl.dropbox.com/u/1315574/IDMap dump 21-2-2013 at 15.8.53.772.txt

As you can see, I only have Vanilla and IC² items.

What the heck is going on here?

EDIT: I now realise that every mod config uses the same method, ie. block { I:name=<id> } and is probably writing directly to the game data, which removes my fear of inconsistencies, for now.

ha, I just came to this realisation and was editing my post before I read your reply, well, at least it is normal.

Can't say for certain until I've done more testing, but the inconsistency is worrying, as it seems to only apply to Items, not blocks, but I have a feeling it's still not that straight forward. I am expecting some mods to handle it internally resulting in wysiwyg whilst others wont.

If anyone is aware of any educational material on the subject, I'd be very greatful.