Trying to import messages to mail, with mixed results...

I have a new laptop, so I'm trying to transfer the Archive folder in Mail (on my old laptop) to the new laptop. Both are running the latest versions of Mountain Lion.

There are 20,000 messages in the Archive. But when I export it, and then import on the new machine, I'm left with only 5000 of the most recent ones.

Here's the process I'm following - am I doing something wrong?

1. Select Archive on old Mac

2. Right click, Export Mailbox, save the mbox file to external hard drive.

3. Plug external hard drive into new Mac.

4. File > Import Mailboxes > Import Data from Apple Mail, select mbox file and Continue.

5. An Import folder is created in Mail, but with only 5000 messages rather than the 20,000 I started with.

Any ideas, friends?