It is very simple, all the packs use the same configs (at least if it comes to blockIDs).
So if the Mod in the DW-Pack uses a certain range of BlockIDs it uses the same range in ultimate or mindcrack or whichever pack.
The FTB-Team propably created a large map of all the IDs and arranged the mods according to this map. Since they will not make multiple ID-maps for several packs it is a lot easier for them to pull all the IDs from the same map and it is easier for us, since we can easily change the modsetup between the packs. The only problem might be that some of the mods have a lot of new items and they need to rearrange some stuff, which hopefully is rather rare, but it could happen and is not exclusive to the ultimate-pack.
The easiest way to confirm is just check the IDs in the Magic, DW and Mindcrack-Packs. You could, if you really want conformation, dig through slows streams, since I asked him this question before, since it makes merging the packs very easy and well he said that it is easier for them to make all the configs use the same ID.
The only exception is Advanced and Compact-Solars, I guess they use the same ID, which actually does make sense.