In early days of the Vocaloid usage, it became a custom for the elite communities in Japan to use the name of a producer of Vocaloid based music with the honorific "-p". This custom was from the arcade game THE iDOLM@STER and the title separated the professional or big producers from the small time amateurs and help identify certain creators within the fandom. When a creator becomes popular at some extent, the title "-p" is given to him by fans. (However, not all the creators use "-p" even if they are given such a title.) While this practice still continues, western producers have also begun to describe themselves "-p" for their own purposes. Sometimes this is not done in Japanese way, since they do not know why this appears on the names of Japanese producers, other times it is simply because it is much harder for non-Japanese speakers to become a producers and essentially earn a "-p" for themselves in the Japanese communities because of the language barrier.
The Japanese producers never give themselves the title and it is always their fans that do this. Even if a creator, who does not own a "-p", wants to release his work under a name, he will not identify himself using "-p". This partly comes from one of the common features of the Japanese, demonstrated in the proverb "能ある鷹は爪を隠す nou aru taka wa tsume wo kakusu" (a shrewd hawk does not show off his talons). In other words, it means "someone with true ability does not need to show off" and modesty is often taken as one of the most valuable things in Japan. A creator usually hides who he/she is at first, and not until does the creator establish a fame on Nicovideo that he/she comes out. If you do not follow this procedure, you would invite the negative misunderstanding as "a show-off who is trying to push himself/herself on Nicovideo" in Japan.