Version hashed twice?

In a v4 signature packet, the packet version number is hashed twice, 
once in the hash of the packet data, and again in the trailer.
Why?

Cheers,

Ben.

--
http://www.apache-ssl.org/ben.html       http://www.thebunker.net/

"There is no limit to what a man can do or how far he can go if he
doesn't mind who gets the credit." - Robert Woodruff