The two guys who wrote this film had a Scary Movie type script while the Wayans and their two friends had a horror spoof as well. Afraid of the competing movies, the studio (who had the Wayans script) also got the other script and they basically put the two together.

From IMDB
The script for Scary Movie is actually a combination of two scripts Dimension Films bought in 1998: "Last Summer I Screamed Because Halloween Fell On Friday The 13th" written by the Wayans brothers and Phil Beauman; and "Scream If You Know What I Did Last Halloween" by Jason Friedberg and Aaron Seltzer. The latter was the working title for this project.

So basically, now you know that all the funny shit in the film was from the Wayans'.