Merge conflicts files that weren't modified

(I'm pretty sure I had already posted this once, but it's been out there for
a while and I still haven't had a response.)

The software we develop can be highly customized at our client's request.
We're attempting to use svn to allow us to maintain these customizations for
our clients, and also allow us to push updates and bug fixes committed to
our trunk system to these client's individual branches.

Let me precept my issue with my understanding of what a merge is. From what
I understood, if you have two branches and you modify them both, a merge is
meant to allow you to copy the changes from branch A to branch B without
overwriting the changes in branch B. Additionally, this only merges the
files that were modified in branch A to branch B and only has a conflict if
the same line in the same file was modified in both branches A and B during
the selected revisions.

Here's my scenario. This is all using Tortoise, mind you:

/branches/client1 created from trunk rev 5
/trunk modified 30 files, rev 7

merge:
from url: /trunk, rev: 5
to url: /trunk, rev: HEAD
destination: /branches/client1
(no conflicts, all goes well, 30 files modified.)

/branches/client1 modified 5 files, rev 9
/trunk modified 50 files, rev 10

merge:
from url: /trunk, rev: 5
to url: /trunk, rev: HEAD
destination: /branches/client1

I get conflicts on 12 files, most of them which were not even modified in
client1 since the last merge!

I can only guess that it's because those files were modified during the
*previous* merge, in rev 8. If this is the case, how can I sell svn, "Hey! I
only need to keep the revisions I've made to the destination since my last
merge."?

-- 
Denny Caldwell
Grinn Productions
www.grinn.net