We did want to retrieve the definitions from your built-in dictionaries when synced Kindle Vocabulary Builder words using Kindle Mate. However due to the technical limitations, it’s impossible (at least for now) to access the dictionary files residing in Kindle, in addition, some of which are drm-protected.
That’s why we use the workaround instead, allowing you to download the definitions from online sources, like WordNet and Bing/Powerword (F12 for the shortcut in Kindle Mate).
We are still working hard to improve this part. Kindle Mate 1.35 supports local dictionary files, which could be used to add definitions to your Kindle words.