This is normal for PAT but you could use the fixup esp-ike but you are limited to one tunnel.
http://www.cisco.com/en/US/products/sw/secursw/ps2120/products_command_reference_chapter09186a00801727a8.html#wp1067379
fixup protocol esp-ike
The fixup protocol esp-ike command enables PAT for Encapsulating Security Payload (ESP), single tunnel.
The fixup protocol esp-ike command is disabled by default. If a fixup protocol esp-ike command is issued, the fixup is turned on, and the firewall preserves the source port of the Internet Key Exchange (IKE) and creates a PAT translation for ESP traffic. Additionally, if the esp-ike fixup is on, ISAKMP cannot be turned on any interface.
sincerely
Patrick