Mailboxes content weirdness - not showing all mails when bringing up mailbox via search

I have a certain mailbox called "CSS and HTML" in my hierarchy of mailboxes.

If I use the left sidebar to locate and enter the mailbox I can see all the conversations I've filed there. There are currently 36 threads there, each with several messages.

However, if I try to get to the mailbox by using this shortcut:

(1) Click in Search.

(2) Enter CSS and HTML.

(3) Select the CSS and HTML item from the "Mailboxes" section of the search results menu.

it's only finding 20 threads - not all the threads I see by going into CSS and HTML directly.

It seems that some threads which are actually inside the mailbox "CSS and HTML" are not actually marked that way. They are just marked as "Archive (all)".

Very confusing.

Why wouldn't selecting that mailbox from the Search results actually show the contents of that mailbox? Why doesn't moving the messages into that Mailbox consistently mark the mails as being in that mailbox?

Thanks,

doug