The proper names word list and the user word list are stored in separate dictionaries on your MessagePad. The Last Name and Company fields in the Name File use the proper names word list to recognize text. When you enter a name into a new name card, the MessagePad uses only the proper names word list to look up and match the text you have entered.
Since the word "Developer" is entered in the user word list, the MessagePad won't recognize it and place it into a name card.
The built-in keyboards look at all word lists to recognize words, so the word "Developer" would be recognized and placed into the Note Pad or Calendar.