This is done to preserve a sense of the original context and flow, especially with regards to subsequent replies. It also makes it impossible for trolls to engineer the appearance of an argument between two innocent third parties, by removing all trace of provocative inserts.
The difference between "Deleted by the poster" and "Deleted by an administrator" is self-explanatory: users can only delete their own posts, individually, and these would then be marked as such. An administrator can delete anything, and these deletes are marked as such.
An interesting exception is the case when a user cancels their IMDb.com account. This also triggers an automated process which removes all their existing posts in the boards. This process runs with administrative privileges. Posts removed in this manner therefore are also displayed as "Deleted by an Administrator".