two files diff clean against head, but not each other

I have gotten two working directories into a state that I don't understand.

I have a branch, and I have a working directory that I've been using for
awhile, and a fresh checkout, both from the branch.

Here is svn info on both files, to verify that they are both in fact from
the branch mybranch:

(~/mybranch/includes)$ svn info HeaderFile.h
Path: HeaderFile.h
Name: HeaderFile.h
URL: file:///usr/apps/svnrepo/branches/mybranch/includes/HeaderFile.h
Repository UUID: f63b3bd2-9f05-0410-b688-c171f8e8e1ab
Revision: 85
Node Kind: file
Schedule: normal
Last Changed Author: alice
Last Changed Rev: 72
Last Changed Date: 2006-02-16 09:37:08 -0800 (Thu, 16 Feb 2006)
Text Last Updated: 2006-02-21 11:18:17 -0800 (Tue, 21 Feb 2006)
Properties Last Updated: 2006-02-21 11:18:17 -0800 (Tue, 21 Feb 2006)
Checksum: d9bce0394d4549e0ba9e090d2814b112

(~/mybranch.fresh/includes)$ svn info HeaderFile.h
Path: HeaderFile.h
Name: HeaderFile.h
URL: file:///usr/apps/svnrepo/branches/mybranch/includes/HeaderFile.h
Repository UUID: f63b3bd2-9f05-0410-b688-c171f8e8e1ab
Revision: 85
Node Kind: file
Schedule: normal
Last Changed Author: alice
Last Changed Rev: 72
Last Changed Date: 2006-02-16 09:37:08 -0800 (Thu, 16 Feb 2006)
Text Last Updated: 2006-03-08 17:23:25 -0800 (Wed, 08 Mar 2006)
Properties Last Updated: 2006-03-08 17:23:23 -0800 (Wed, 08 Mar 2006)
Checksum: d9bce0394d4549e0ba9e090d2814b112

Both files diff clean against the head of the branch:

(~/mybranch/includes)$ svn diff -r HEAD HeaderFile.h

(~/mybranch.fresh/includes)$ svn diff -r HEAD HeaderFile.h

However, the two files are not identical. They do not diff clean against
one another:

(~/mybranch/includes)$ diff
HeaderFile.h../../mybranch.fresh/includes/HeaderFile.h
83c83
< const Variable *const var,

---
>       const Variable<double> *const var,
176c176
<    const Variable *const d_var;
---
>    const Variable<double> *const d_var;
Can someone explain to me how this is possible?
Thanks,
Bob