Hi Bellan,
Yes, it is true that the "Word Split" categories in the English content contain the same words from the "General" categories. If you open the Edit Word window for "dog" from General Categories > Common Animals 1, you'll see that the pronunciation audio for both the "normal" (whole-word) and "split" versions are included in the Word Slide tab - which means the "dog" word you see in the General and Word Split categories is actually the same word file.
The difference is that the General category has a playback setting where "Split Audio" is turned off, while the Word Split category has "Split Audio" turned on. That's the main reason why there's a separate category specifically for Word Split versions.
To make things easier, you can simply select a playback preset for your word split category. LR v3 includes a handful of playback settings that you can use - "Multisensory" is for general use, "Multisensory (Split Colors)" shows words in color-coded parts, and "Multisensory (Split Mode)" shows words in color-coded parts and also with split audio.
You can also create your own playback presets or just change the playback settings for that particular category. Note that the split options are found in the "Advanced" tab, beside the "Display Mode" label.
I hope this helps!